Poland and Ottomans should be set as Historical Enemies, or something. Poland can continually get events that give them claims against the Ottomans, along with bonus manpower boosts. Their bloody King just died in battle against the Ottomans at the start of the campaign. That's THE event marking the beginning of EU4! (that is why Bohemia, Hungary, and Poland all start in Interregnums) So why are they always giving the hated Turks access through their country?
Yeah, it's annoying. It really, really messes with controlling the Bosphorous sound, which is what anyone with interests in the Balkans (Venice, Byzantium, Serbia, etc.) has to depend on. Poland really should be hating them; be rivaling them.
As for the rest, it's because the AI doesn't pay a diplo cost for military access. They can and will get access to every single nation bordering anyone they are at war with or who is along the path to whoever they are at war with, because there is no downside to them getting it. It's basically a crutch - an understandable one - because to teach the AI how to get access and move through areas as it needs them, then cancelling access, would be unreasonably hard to code