1 - Any bridges would be supplied by third parties - you can check here: https://xenforo.com/community/resources/categories/add-ons-2-x.44/
2 - Yes.
3 - No, XF uses responsive design, rather than an app - although I believe there may be third party apps available.
4 - Yes.
5 - Yes.
You can set the 'Receive new conversations' permission to No for the Registered user group, and set it to Yes for the staff user group.
Note that only refers to new conversations from that point on - it won't affect existing conversations.
The importer has only been tested with up to VB 5.3: https://xenforo.com/xf2-docs/manual/importing/
It may work for 5.5, but it depends on whether any schema changes have been made between 5.3 and 5.5.
You could try a test import and see how it works.
I don't see a recent purchase/renewal against the license associated with this account.
Are you sure you logged in to the correct one?
You can submit a ticket here to discuss it: https://xenforo.com/contact/