I'm confused about how to translate 帰路についてくれた.
Basically, the speaker is thanking the people for going home.
The problem with translating these kinds of Japanese phrases is that if you try to translate everything in a Japanese text into some kind of English equivalent, you end up with unnatural expressions. The person is already thanking the people for ...
what if I didn't want that someone to eat the food, but that someone still ate the food (and I am pissed about it).
How about 「食べやがった(な)。」「食べやがって！」, using 「やがる」?
You can also say 「よくも食べやがったな。」「よくも食べやがって！」 or just 「よくも食べたな！」
(But please note that 「～やがる」 and 「よくも」 both sound pretty strong, angry and rough.)
will it be possible to use the same sentence ...
You can say 私が手紙を送ったのは彼だった.
This 私が is more or less important. If you omitted 私が, the sentence would become ambiguous:
He is the one who sent a letter (to someone).
He is the one I sent a letter to.
The use of あげる cannot solve this type of ambiguity (手紙を送ってあげたのは彼だった is still ambiguous the same way). And even the following ...