My dear Gentlewoman
This sounds too horrific to be true. I don't think I've ever heard of anyone bringing a plastic container to a dinner party - it sounds far too dramatic if not extremely impolite. Of course, there are instances when someone is truly allergic, but surely the host or hostess would be advised in advance. I always subscribe to the philosophy of treating others as you would expect to be treated. So, yes, a guest should not only be grateful, but also be discreet if there are certain issues or dislikes. As you suggest, pushing food around one's plate sounds quite reasonable and diplomatic.
In my opinion, the only time guests should specify a particular issue is when there are actual health issues involved. Perhaps it sounds rather cold, but if you encounter these fussy sorts (you know, drama queens or fad diet addicts), perhaps it is best not to invite them back -- I know this is a touch extreme, but surely you want to enjoy your own dinner party. It is supposed to be a lovely and special affair with guests, not an evening filled with annoyance and drama (well, not that type of drama anyway).
Hope this helps, and may all your future dinner parties be trouble-free.
Fondly
Prunella