Hi again Orbiter! You are such a great student of Korean! I should be doing my homework but I'm burned out at the moment --your questions are providing me a little much-needed distraction.
1)"...거고"
- "...있는 거고"; "...되는 거고"; "할거고"; "볼거고" etc. Is this from "것이다/거다"? (By the way, is 것이다 and 거다 the same? They both imply the future right?)
This looks to me like a contraction of ~것이다. 그리고... the idea that something (거) IS (있는), turns out as (되는), will be performed (할), will be tried (볼) .. and that the information is not complete (고) 'and something' could be added/ additional explanation or information --but some people just end there with "고" as a kind of speech habit.
And as far as I know 것이다 and 거다 have the same meaning, also, I think the pattern is only be future tense when preceded by ~ㄹ 있을 거다, 될 거다, 할 거다, 해 볼 거다 etc.
2) "[thought/speech] 하다"
- "경찰에 신고할까 하다가, 무슨 일인지도 모르고해서..." (this pattern is used twice here, first with 하다 then with 해서... right?)
- "이건 아니다해서 글올립니다"
Is this the same as "[speech] 싶다" e.g. ""경찰에 신고할까 싶다"; "그 사람은 부자일까 싶다"; "친구한테 연락 하는게 낫지 않을까 싶은데..."
Those look great to me. Yeah I they are nearly identical in meaning "thinking about but hesitating" Orbiter I think you understand these better than I do, I'm always getting this form of ~~싶다 mixed up with ~~고 싶어요.
3) "~다 마나"
- "왜 밥이 먹다 마나?" (Why did you eat and then stop eating? Something like that)
- "말을 왜 하다 마나?"
I guess this is used in a similar way as "할까 말까". Is this usage formal/informal?
The pattern you are looking for is probably ~나 마나 and expresses "there's no point in"
하나 마나 - "there's no point in doing something"
먹으나 마나 - "there's no point in eating it" usually meaning "It's too tiny a portion"
그 약은 먹으나 마나였다. There was no point in taking the medicine (it didn't help at all).
- "왜 밥이 먹다 말다 하니?" Why did you eat and then stop?
- "말을 왜 하다 말다 하니?" Why did you talk and then stop talking?
I think 할까말까 is only used in the first person -- like "I'm wondering if I should or shouldn't."