Fortunately with allium issues the more it is cooked, the more the proteins break down. Garlic isn't as bad as onion or leeks or shallots. Green onion stems aren't as potent as well and chives seem to be similar. It's all about how potent that protein is. So if those cooked ones touch my food it's fine. I can even tolerate garlic powder without issue (it's effectively getting rid of the proteins that cause the issues). I like the taste of it, but can't do it too much and it needs to be cooked. Onions are far bigger of an issue. Sautéed onions aren't enough, they need to be reduced and removed for me to be okay. One bite of cooked I can deal with. One bite of raw or raw touching my food and it's bad.
But yes, the kitchen contamination is a concern if highly allergic (I am not, it's the level of a strong intolerance, never life threatening but not something I want to deal with on vacation). However the food dropping on each other or utensils touching etc that all makes me avoid most buffets unless I know they aren't onion heavy foods.