1. 8 CFR Article 245.2(a)(4) states AOS applicant who is in lawful H-1 or L-1 status shall safely travel out of the country, upon returning to this country, the alien remains eligible for H or L status, is coming to resume employment with the same employer for whom he or she had previously been authorized to work as an H-1 or L-1 nonimmigrant, and, is in possession of a valid H or L visa (if required). You will be in H1B status as you entered with that status and also your I485 application is still valid. As an additional bonus, it also triggers Sec 245K protection, which essentially forgives you for previous status violations.
2. Two scenarios: a) If the dependent use EAD for employment they have to use AP to enter. Any underlying H4 / dependent status will cancel. b) If the dependent uses AP only for entry the above clause will prevail. i.e., they will still keep underlying H-4 status. In both cases, as primary entered on H1B and still maintaining H1B, only the dependent will move to "Parolee". I-94 status will reflect "DA / Parolee".
3. As someone had used AP, their I-94 will expire 1 year from the date of entry. However, they will still be in "AOS Pending" status after that date.
These AP related questions are very tricky as you are trying to use multiple benefits / statuses without cancelling one of the them by mistake.
I see a lot of questions lately on EAD and AP. A lot of good info is already available in threads. Please take your time to grasp existing info: