1. The difference between 하던 & 했던
As far as I know, there's no difference between 하던 and 헤던. I guess both of 'em can be used in the same context, having no change in meaning. Actually, it sounds a little weird and unnatural to use 던 in the present tense, I've never seen it before. Koreans never say ' 내가 하던 일이에요', they say '내가 했던 일이에요'. (That's the work I did)
하다(present tense) + 던 = 하던 - Almost never used in every day speech, maybe in some kind of dialect.
했다(past tense) + 던 = 했던 - Sounds more natural.
2. The difference between 했었어 and 하던
Well, the former one is the Past Perfect tense of Korean language. Expresses a past occurrence that doesn't continue to the present.
현우 씨는 작년에 한국에 왔았어요. (Hyunwoo came to Korea and later left; he is not in Korea now)
The latter one has already received an explanation. You cannot compare those two because they're different grammar points, one can never replace the other.
3. 은/ᄂ and 했던 in forming adjectives in the past
I don't know what you actually need an explanation for...
내가 가는 곳이에요. - That's the place I go to. (는 particle, used to form adjectives related to present actions)
내가 갔던 곳이에요. - That's the place I went to. (았던/해던, used to form adjectives related to past actions)