Even though your remaining on the same realm, I believe you won’t be able to transfer while being guild leader. If you can transfer it to one of your alts, that might be best, then you can transfer back after it is complete.
You can buy a transfer bundle, you will usually get a discount, but the best way is to wait for a character service sale. They happen a few times a year, so if you can, wait.
It depends. Some Boosts are bound to the wow license, some are to the Battlenet. If you can see the boost when logging into either license, you should be able to use it from either.
Possibly. If a character hasn’t been logged into for awhile you may need to log in prior to being able to transfer it.
Most mounts/pets are bound to the Battlenet account, not a specific license, so if you can use the mount/pet on either, then it is definitely available to the account.