I'm glad the cross-post date still works -- I can look into the DST problem.
Approval suggests to me that either your user isn't integrated as you think, or it is integrated but not being logged in in phpBB while you are in the WordPress admin.
For the first possibility: Check the WP-United user mapper to ensure your wordpress user is mapped to the correct phpBB user, and then check in phpBB that you are logged in correctly as this user.
If you had user auto-create turned on from the outset, WP-United probably created a new user for your wordpress user before you had the chance to map together existing accounts. If that is the case, then turn off user auto-create, then visit the user mapper and clean things up, mapping together existing accounts in phpBB and WordPress until things look good (you only need to worry about pre-existing accounts that exist in BOTH WordPress and phpBB -- they should be mapped together. Everyone else can get auto-created counterparts.)
When everything is clean in the user mapper, then you can turn back on auto-create and make sure that permissions are mapped correcty so that future users are auto-created properly.
If your user is mapped properly, then I'd consider the second possibility: phpBB may not have considered you to be logged in properly at the time you posted. WP-United should be handling that for you -- the first place to look in that instance would be at your phpBB cookie settings -- ensure the server and path settings cover your entire site, including the WordPress admin.
I'm not sure why that permissions setting wouldn't be retained, I've never met this issue. As far as I understand, if the permissions showing up in the panel, it's in your auth_acl_options table which is enough -- You're looking at the permissions for specific forums, right? And clicking the apply button at the bottom of the screen? If the problem persists I can come on and take a look at your setup.
Cross-posted comments are managed by the phpBB WP-United permission. So if a user has that, they can comment in WordPress, even if the normal phpBB guest posting permissions are disabled for that forum. Their username will appear as whatever they typed in in the WordPress form, with characters appended to it if it matches an existing phpBB username. WP-United can also pass the comment through e.g. Akismet, which is obviously recommended.
Thank you in advance with help with the translation!