Papal resignations and participation in subsequent conclaves If a pope abdicates, after his successor's death, would he able to participate in the conclave, and if chosen is there any ruling to prevent him from being able to be pope again?

The only examples I can find of popes having more than one spell are Benedict IX (whose abdications and returns to power were more than a little unconventional) and antipopes who are elected as pope after their challenger has died, but there's no examples of this for a legitimate pope.
There is very little in the way of restriction on who can be pope. Canon law simply says that he must be consecrated a bishop before taking power; but it allows a non-bishop to be elected, providing that he is ordained a bishop before he takes his place ("If the person elected lacks episcopal character, however, he is to be ordained a bishop immediately", Canon 331 section 1). But neither canon law nor Universi Dominici Gregis, the current Church document governing the election of a pope, fixes any further limits on who may or may not be elected. 

In fact Universi Dominici Gregis goes further. Its paragraph 35 reads in part, 


  No Cardinal elector can be excluded from active or passive voice in the election of the Supreme Pontiff, for any reason or pretext. 


In this context, "active voice" refers to the right to vote for pope, and "passive voice" to the right to be voted for. No one who is an elector can be made ineligible to be elected. 

Note that paragraph 35 does not strictly apply to Pope Emeritus Benedict, who is over 80 and therefore ineligible to vote for pope. UDG (as it is sometimes abbreviated) states in its prologue:


  those Cardinals who celebrate their eightieth birthday before the day when the Apostolic See becomes vacant do not take part in the election. 


It follows that this particular pope emeritus could not be a cardinal elector and thus does not have this specific inclusion. Nevertheless, especially in view of the oath each Cardinal must take to vote for the one that believe to be the best man for the job, he's still not hindered or prohibited from being elected pope.