「それぐらいのこと気にすんな。このまますぐ帰るのもなんだし。飯でも奢ってやろうか」
What exactly is the meaning here? He's telling the other person "Don't worry about this much" and the then "let me at least offer you lunch?"
このまますぐ帰る is "like this you'll go home immediately". What nuance does のもなんだし provide?
It's not ものだ given that it's のもなんだ, so?
For context, they were going to visit someone but it turns out he wasn't home. So the narrator apologizes for wasting his friend's time and tells him this.
なんだし is derived from 何だし, and here なに is sort of an euphemism/placeholder you can use to refer to something without precisely describing it. It usually refers to something slightly negative / problematic / unnatural etc. (generally something negative).
In this context, the speaker is saying something like "it wouldn't feel right if we go home now", "it would be inconvenient if we go home now".
By adding the し, it adds the meaning of "therefore".
In this specific context, without knowing anything else I'd guess that the other person had maybe a rough day, had something hard happen to them or something so that the speaker feels they should give them an opportunity to talk/decompress etc.
なんだし is a common pattern when using なに as euphemism, but you can also use it in different ways. Like 起こしちゃったらなんだから、アラームは切っておこう or 遅刻したらなんだぞ。
の is nominalizing the このまますぐ帰る and the explanatory なんだ with the し particle is listing it as a reason.
Literally: "There's also the fact that you're going home like this immediately, so I might as well buy you a meal."
I find it useful to translate japanese word to word like this, sounds weird in english, but helps with understanding the sentence.
