How does Roman Catholicism justify a literal meaning of 'Hoc est meum corpus'? The words :-

τουτο εστιν το σωμα μου το υπερ υμων διδομενον [Stephanus 1550/Elzevir 1624]
this is my body which is given for you [KJV 1769 Luke 22:19]

and, particularly :

hoc est meum corpus [Vulgate - Jerome 382/Clementine 1592]
this is my body

were spoken as Jesus stood before the disciples, his hands holding out bread to them.

How then, at that instant, could bread have been - literally - his own real body. ?

It was not - literally - true that the bread was 'given' (sacrificially) at that instant of time. It is a figure of speech. It is the way concepts are expressed in language.

And if Jesus requires



this do in remembrance of me [KJV 1769 Luke 22:19]

then it is a memorial of what was about to occur. Not a physical re-enactment. It is the faculty of memory that he wishes to be engaged, not the process of digestion.

How are these particular questions answered by those who suppose a literal meaning to hoc est meum corpus ?
I will attempt to answer your questions, but I think we need to start with a detour into what the Catholic doctrine of the Eucharist actually is. You use a couple of words that lead me to think you misunderstand the doctrine: &quot;digestion&quot; and &quot;literally ... real&quot;.
The Catholic doctrine is not that the Eucharist is Christ's body in the same way that you could get a sharp knife and cut a bit of him off. (This isn't The Merchant of Venice!) The doctrine is that

In the most blessed sacrament of the Eucharist &quot;the body and blood, together with the soul and divinity, of our Lord Jesus Christ and, therefore, the whole Christ is truly, really, and substantially contained. (Catechism of the Catholic Church, ¶1374)

Similarly, we can read Thomas Aquinas, who answers the question &quot;Whether Christ's body is in this sacrament as in a place?&quot;

Hence it remains that Christ's body is not in this sacrament as in a place, but after the manner of substance, that is to say, in that way in which substance is contained by dimensions; because the substance of Christ's body succeeds the substance of bread in this sacrament: hence as the substance of bread was not locally under its dimensions, but after the manner of substance, so neither is the substance of Christ's body. (Summa Theologia III, 76.5)

That is to say, Christ's presence in the Eucharistic species is not restricted to the Eucharistic species. It isn't &quot;a bit of Christ&quot;. It is just &quot;Christ&quot;.
I think that answers your first question: &quot;How then, at that instant, could bread have been - literally - his own real body?&quot;
You next say that the bread was not &quot;given&quot; at that moment of time. Well, yes: the sacrifice in question is the sacrifice of Christ on the Cross, of which the Last Supper is part and of which the Eucharist is a re-presentation. Again, from the Catechism:

The sacrifice of Christ and the sacrifice of the Eucharist are one single sacrifice (Catechism of the Catholic Church, ¶1367)

Or, from Sacrosanctum Concilium:

At the Last Supper, on the night when He was betrayed, our Saviour instituted the eucharistic sacrifice of His Body and Blood. He did this in order to perpetuate the sacrifice of the Cross. (Sacrosanctum Concilium, ¶47)

The Eucharist is intimately related to the Cross: they are one sacrifice.
Finally, you talk about the question of a &quot;memorial&quot;, which you contrast to a &quot;physical re-enactment&quot;. The Catholic doctrine is that &quot;memorial&quot; means a lot more than a simple &quot;remembering&quot;. Really it's worth reading the entire section of the Catechism entitled &quot;The sacrificial memorial of Christ and of his Body, the Church&quot;. Here are a few key parts:

The Eucharist is the memorial of Christ's Passover, the making present and the sacramental offering of his unique sacrifice, in the liturgy of the Church which is his Body. ...
In the sense of Sacred Scripture the memorial is not merely the recollection of past events but the proclamation of the mighty works wrought by God for men. In the liturgical celebration of these events, they become in a certain way present and real. This is how Israel understands its liberation from Egypt: every time Passover is celebrated, the Exodus events are made present to the memory of believers so that they may conform their lives to them.
In the New Testament, the memorial takes on new meaning. When the Church celebrates the Eucharist, she commemorates Christ's Passover, and it is made present the sacrifice Christ offered once for all on the cross remains ever present. (Catechism of the Catholic Church, ¶¶1362ff)

So the Catholic view is that a memorial is a much richer thing than you suggest. The Eucharist is about presence, memorial, sacrifice, communion, and a whole host of other themes.