よかったぁ！ お祝いで 今日はごちそうなんだから！！
With this sentence I wonder would this mean something similar to ''Because we'll be having a feast today as a celebration'' considering that 男の子で東京に行く Can mean ''Will go to Tokyo as a boy'' ?
I'm merely asking as it seems both で and に can mean ''as'' in certain contexts. So how do you distinguish between those and when and why can で mean ''as'' a state of something?
What do you mean by "go to Tokyo as a boy"? Is this about a person who can freely change their gender every day? If such a context has been already set up, 男の子で東京に行く might make sense, but otherwise it's a puzzling sentence.
it seems both で and に can mean "as" in certain contexts. So how do you distinguish between those
に is a role marker. If your "as" means "serving as" or "functioning as", you can use に.
で is a condition/scope/reason/method marker, and it corresponds to various English prepositions ("in", "at", "by", "with", ...) depending on the situation. You cannot expect one-to-one correspondence with English prepositions at all, so you need to remember the correct usage individually. See this: Does で in this sentence 「日本の昔話は代表的なものだけで、２，３百はあります。」 mean that the noun before it is a tool for the proceding sentence?
Provided that this person has the special ability to change gender, 男の子で東京に行く makes sense (it describes their state when going to Tokyo), but 男の子に東京に行く makes no sense ("serving as a boy" means nothing here).
