1. You would need to check your cookies here.
2. the photopost login page is misc.php so sorry not an issue you cant compare that to a vb
3. I highly suggest you make sure when you installed you correctly deleted all usergroups before reinstalling to integrate with xenforo as I would suspect your group permissions are not correct.
4. There is no fix the way its coded is how its released A community photo gallery - My Photo Gallery
This is my little test site works fine with xenforo logging in either way. You only have two real issues posted above thats your usergroup permissions and being cookied in. Both issues can be explained by not following the exact full steps above posted about switching integrations.
Just my opinion but I really cant say more without physically seeing your install.