1. I'm not sure what the "group" is exactly, but I assume the students are supposed to form some group and then one by one "insert" people into it who've done homestays in Japan.
が can be replaced by の in relative clauses with just a subject and verb, without a difference in meaning. This comes from older Japanese (don't remember which era... classical, maybe?) where their roles were reversed. You can still see it in phrases like 我が社, 我が家, 我が国, and in place names like 関ヶ原. All the above cases are が as a genitive marker, like の in 私の家.
2. "(The) Japanese and Slippers"
3. ~といったところだ is a set expression. It means something like "around the area". Well... uh... it's not so easy to put into English.
4. Yeah, but I think か should go with どうなる, i.e., どうなるか ("what will happen" or "how it will turn out"). That's what the worry is about.
5. "Students who study well".
6. "Also, it may be that the feeling that just because you study doesn't mean you'll have a good future is getting stronger." 必ずしも is a negative polarity item (
NPI) and means "not necessarily".