Who oversaw the 9th, 10th, and 11th sessions of the Council of Trent? From the wiki page on the Council of Trent, we have:


  Pope Paul III, who convoked the Council, oversaw the first eight sessions (1545–47), while the twelfth to sixteenth sessions (1551–52) were overseen by Pope Julius III and the seventeenth to twenty-fifth sessions (1562–63) by Pope Pius IV.


Was there an overseer for those sessions not presided over by a Pope? Why weren't those sessions overseen by a Pope?
Session 8 voted to move the council to Bologna, due to the political situation of Germany / Emperor Charles V.

Celebrated on the eleventh day of March, 1547
DECREE CONCERNING THE TRANSLATION OF THE COUNCIL
Does it please you to decree and declare that from the foregoing and other reports regarding that disease, it is so clearly and manifestly certain that the prelates cannot without danger to their lives remain in this city, and that therefore they cannot and ought not to be detained in it against their will? And considering, moreover, the withdrawal of many prelates since the last session, and the protests made in the general congregations by many other prelates wishing by all means to depart from here through fear of the disease, who cannot justly be detained and by whose departure the council would either be dissolved or, from the small number of prelates, its beneficial progress hindered; and considering also the imminent danger to life and the other manifestly true and legitimate reasons alleged in the congregations by some of the Fathers, does it please you likewise to decree and declare that for the preservation and prosecution of the council, and for the safety of the lives of the prelates, this council be transferred for a time to the city of Bologna as a place more suitable, more healthy and better adapted, and that the translation have effect from this day forth, that the session already announced for the twenty-first of April ought to be celebrated and be celebrated there on the day appointed; and that further matters be proceeded with in succession, till it shall seem expedient to our most holy Lord and to the holy council, with the advice of the most invincible Emperor, the most Christian King, and with the other Christian kings and princes, that this council may and ought to be brought back to this or to some other place?
They answered: It pleases us.

Sessions 9 and 10 were

Celebrated at Bologna on the twenty-first day of April, 1547;
Celebrated at Bologna on the second day of June, 1547,

respectively.
Julius III issued a bull on 14 December 1550 to reconvene the Council at Trent:

In order to put an end to the religious dissensions which for a long time have prevailed in Germany to the disturbance and scandal of the entire Christian world, it appears good, opportune and expedient to us, as also to our most dearly beloved son in Christ, Charles, ever august Emperor of the Romans, who has made this known to us by his letters and ambassadors, to bring back to the city of Trent the holy ecumenical and general council convoked by our predecessor, Pope Paul III, of happy memory, and begun, conducted and continued by us, who then enjoyed the honor of the cardinalate and, conjointly with two other cardinals of the holy Roman Church, presided in the name of our predecessor in the council, in which several public and solemn sessions were held and several decrees promulgated on the subjects of faith and reform, and also many other things relating to both subjects were examined and discussed. We, therefore, to whom, as reigning sovereign pontiff, it belongs to convoke and direct general councils, that we may, to the praise and glory of Almighty God, procure the peace of the Church and the increase of the Christian faith and of the orthodox religion, and may, as far as we are able, consider with paternal solicitude the tranquillity of Germany, a province which in times past was second to none in Christendom in cultivating true religion and the teaching of the holy councils and the holy Fathers, and in exhibiting due obedience and reverence to the supreme pontiffs, the vicars on earth of Christ our Redeemer, hoping that by the grace and bounty of God it will come about that all Christian kings and princes will approve, favor and aid our just and pious wishes in this matter, by the bowels of the mercy of Our Lord Jesus Christ, exhort, command and admonish our venerable brethren the patriarchs, archbishops, bishops, our beloved sons the abbots and each and all of the others who by right, custom or privilege ought to be present at general councils, and whom our predecessor in his letters of convocation and in any others made and published with regard to this matter wished to be present at the council, to convene and assemble in the same city of Trent, since the lawful impediment no longer exists, and to apply themselves without delay to the continuation and prosecution of the council on the next first of May, which day we, after mature deliberation, of our own certain knowledge, with the plenitude of Apostolic authority, and with the advice and consent of our venerable brethren, the cardinals of the holy Roman Church, appoint and set aside for the resumption and continuation of the council in the state in which it now is. We shall make it our special care that our legates will be in the city at the same time, through whom, if on account of our age, state of health and the demands of the Apostolic See we shall be unable to be present personally, we shall under the guidance of the Holy Ghost preside over the council; any translation and suspension of the council and any other things whatsoever to the contrary notwithstanding, and especially those things which it was the will of our predecessor should not create any obstacles, as expressed in his letters aforesaid, which, with each and all of restrictions and decrees therein contained, we wish and decree to remain in force, and so far as there is need we hereby renew them; declaring, moreover, null and void whatever may be attempted knowingly or unknowingly by any person or by any authority to the contrary. Let no one, therefore, infringe this our letter of exhortation, summons, monition, ordinance, declaration, renewal, will and decrees, or with foolhardy boldness go contrary to it. But if anyone shall presume to attempt this, let him know that he will incur the indignation of Almighty God and of His blessed Apostles Peter and Paul. Given at Rome at St. Peter’s in the year 1550 of Our Lord’s incarnation, on the fourteenth of December, in the first year of our pontificate.

Session

celebrated under the Supreme Pontiff, Julius III, on the first day of May, 1551

Quotes from: Schroeder, O.P.'s Canons and Decrees of the Council of Trent
Period at Bologna
From the Catholic Encyclopedia on the Council of Trent:

Before this session [the 8th] was held the question of the prorogation of the council or its transfer to another city had been discussed. The relations between pope and emperor had grown even more strained; the Smalkaldic War had begun in Germany; and now an infectious disease broke out in Trent, carrying off the general of the Franciscans and others.

and

The majority of the fathers of the council went with the cardinal legates from Trent to Bologna; but fourteen bishops who belonged to the party of Charles V remained at Trent and would not recognize the transfer. The sudden change of place without any special consultation beforehand with the pope did not please Paul III, who probably foresaw that this would lead to further severe difficulties between himself and the emperor. As a matter of fact Charles V was very indignant at the change, and through his ambassador Vaga protested against it, vigorously urging a return to Trent. The emperor's defeat of the Smalkaldic League increased his power. Influential cardinals sought to mediate between the emperor and the pope, but the negotiations failed. The emperor protested formally against the transfer to Bologna, and, refusing to permit the Spanish bishops who had remained at Trent to leave that city, began negotiations again with the German Protestants on his own responsibility. Consequently at the ninth session of the council held at Bologna on 21 April, 1547, the only decree issued was one proroguing the session. The same action was all that was taken in the tenth session on 2 June, 1547, although there had been exhaustive debates on various subjects in congregations. The tension between the emperor and the pope had increased despite the efforts of Cardinals Sfondrato and Madruzzo. All negotiations were fruitless. The bishops who had remained at Trent had held no sessions, but when the pope called to Rome four of the bishops at Bologna and four of those at Trent, the latter said in excuse that they could not obey the call. Paul III had now to expect extreme opposition from the emperor. Therefore, on 13 September, he proclaimed the suspension of the council and commanded the cardinal legate del Monte to dismiss the members of the council assembled at Bologna; this was done on 17 September. The bishops were called to Rome, where they were to prepare decrees for disciplinary reforms. This closed the first period of the council. On 10 Nov., 1549, the pope died.