1) Tuna: It would never, ever, ever occur to me that tuna is an ingredient that requires stretching. Canned tuna (to me) is the cheapest type of protein one can get. When we run out of cat food, the cat often gets tuna and the price is not that different. I also buy more expensive eggs so the price differential between tuna and eggs is not huge.
I might not ask to have the egg left out because of an "eat cheerfully what you are served" mentality, but with a close friend, I would ask for no egg.
2) Lobster: Assuming I asked for no celery, I would expect to get a slightly smaller portion (same amount of lobster as I would have with the celery) of the lobster.
3) Roast: I would not look to amount of roast versus side dishes, but amount of roast versus people at table and take an appropriate portion.
Just because I think it's fun: If there were 5 people and one roast, I know that most people I have dinner with would take 1/10 of the meat on the first round. Then people would have seconds and thirds, but NO-ONE will take the last slice.