~snip~
Also, I have another wallet file that I tried to load in Bitcoin Core but was rejected. It might be a wallet of another cryptocurrency (maybe Dogecoin??). How can I check this?
Keep a copy of the DAT file and try with Dogecoin. If it does not work then try with another possible wallet you have used before. And I don't think I understood your first query.
So far the only positive balance was found in one of the "sending addresses" in Bitcoin Core but if I understand correctly, this is not an address I own but an address I sent money to, correct?