1. You need to declinate the noun. In the first case, it is "dopełniacz" case - answers the question "kogo? czego?" In the second you have "biernik" - "kogo? co?". But I am not sure at first I thought that in the first sentence is also "biernik".
Why is this I can not tell, unfortunately.
2 "bym wiedział" - is extremely seldom. They do not mean the same thing.
wiedziałbym = "If I studied more, I would know that thing"
Gdybym uczył się więcej, wiedziałbym to.
gdybym wiedział - "If I would know that, I would not go there"
Gdym to wiedział, nie poszedł bym tam.
I am not sure if my English is good enough to explain it.
3. Yes, they are interchangeable. However, one may be incorrect, but they are equally used.
3a (you have two questions numbered 3
) - there is no obvious difference between them
4 wówczas = wtedy, but I would not say that it is old-fashioned. I use it quite often in formal texts.
5 zaś = natomiast = while / on the other hand - depending on the context. On the other hand is "z drugiej strony" or "z drugiej zaś strony" or "natomiast z drugiej strony"
6 - odejść =/= wyjść
odejść may mean to die:
On odszedł w zeszlym roku
may mean to break from relationship:
Po dwóch latach odeszła od niego
Or simply to walk away
While "wyjść" means to exit:
Wyszedł z kina w trakcie seansu
So your sentences do not mean the same thing (besides you used wyjechać not wyjść) - in the first case you say that he just get into the car and drove away - to work, shop or whatever. But it is completely normal and he is going to come back soon. In the second sentence, however, you are pointing out that his absence will be longer or even he is not going to return. He could go for a trip, holidays etc.
7 - no. In first one it means "they went" and "they approached" in the second one. By the way, the second one makes no sense - it could be "Grupa byłych uczniów podeszła do swojego dawnego profesora"
8 - only with some loss of information - "podejść" is to approach on foot, while "podjechać" is to approach with some vehicle (bike, car)
but "zbliżyć" has also a counterpart "zbliżać" (like present simple) so:
podejść/podjechać - zbliżyć
podchodzić/podjeżdżać - zbliżać
But "zbliżyć" is a dangerous verb. "Doszło międzynimi do zbliżenia" means that the had sex
(in this sentence it is not a verb but a noun derived from a verb - verbal noun)
Hope this will help you at least a little
(edit)
I would propose to change from "skały" to "kamienie" in the first question. "Skała" is big, even as big as a mountain.
Znalazł wiele ciekawych kamieni
Zapakował kamienie do plecaka