As you said, I don't think I can say much more either. Sounds like you guys are trying to figure things out....that's the important part is that you are still working actively to find a solution.
Tib.post. can be a real "trouble-maker" when there are changes to footwear ( or not wearing any) but it should improve. It is a muscle that is actively involved in supporting your arch so as it strengthens up and gets used to its new role with the orthotics it SHOULD improve. Since the orthotic is soft, the pressure from it shouldn't be further irritating the nerves in the area (I wouldn't think so anyway).
All I can say is... it sounds like you're on the right track... but it's a process. Your requirement for increased walking in Poland makes it a wee bit more difficult but not impossible. Boots will help; although your feet are getting used to the orthotics, they will help; hiking poles may help if you are able to use them.
Just want to add one more thing... my cousin has a history of anorexia which is when I learned a little about it...I don't pretend to know alot. All I can say is you are to be commended for your goal and I wish you the best of luck with that goal as well as the issue with your feet. Take care!!