We use “I" or "we” for [STRIKE]“[/STRIKE]instant decisions[STRIKE]"[/STRIKE]:
That's true. We use them for lots of things. But there is nothing special about instant decisions.
1) I’ll have a big bowl of beef noodles, please.
2) We’ll have two hamburgers and two large cokes.
I don't understand why you think that "I" or "We" has anything to do with instant decisions. The timing of a decision is irrelevant to the choice of pronoun.
Both your examples could be used when someone has made an instant decision but it's equally possible that they've been pondering the menu for ages before actually ordering.
Yes, when we are talking about who is making the decision; not always if we are talking about the grammatical subject of the verb.
Damn! I've got meetings scheduled tomorrow in two different places. What shall I do? I know! I'll go to the Paris meeting and John'll go to Berlin for me.