I think it's doable, but you will have to watch your time carefully. Are you walking? Taking cabs?
If you like cemeteries, you might find it interesting but time consuming. I took the tour in English and as I recall it was maybe 45 minutes? Also I think it's a bit of a hike from the Aventino to the Protestant cemetery. If I were you, I would be prepared watch the time carefully. You certainly don't want to miss your visit to the Borghese Gallery. It can take some time to get from Testaccio to the Borghese.
I haven't been to the new Testaccio market so I don't know about the food options there for lunch or whether or not a quick lunch is possible. The Volpetti "deli" is a great spot for cheese and salami. I don't know if they make sandwiches, but their food quality is really good. I understand that they are under new ownership and the space around the corner which used to be a tavola calda (cafeteria) is now a tavern with table service, so likely good food, but also more time consuming than the old place. Volpetti is not in the new market, but it's close and on via Marmorata 47.
I also try to plan extra time to do anything in Italy - it always just works out that way. Also it's likely to be quite warm so you may be walking more slowly than expected.
