You may be able to get a gate agent or one of the RCC agents to change your MP numbers over to AC, but the terms of the E+ upgrade package is quite clear about mileage assignment, unlike booking E+ as a UA elite. That's how I do it (fortunately out of the blue gifted Premier late last year by a generous FTer), and can always get my miles directed into my AE account if I wish. Having requalified for AC Elite months ago, I have just kept my UA flights going into my MP account, and I should hit PremEx in the fall.

Only an agent can ensure that your seating does not get bounced into the back back cabin once your UA number is removed from the file. In fact, you may find that if you can get this done on the outbound, your return seats have automatically gone into the back back coach cabin and you'll have to get E+ seating reassigned at check-in. Unless there is a valid UA elite/E+ number on the res, it defaults to regular coach seating, randomly assigned. Since your first check-in outbound will be by an AC agent, if s/he changes the account number, it could default your transAt segments to the regular coach cabin and you might not be able to get them back until you get to ORD where UA has full control again.

One other thing you can try but it must be done within a day or so of when your flight(s) post to your MP account. Call the MP centre and ask the agent to remove the credit from your account as you wish to submit the flight to AC. This can sometimes be done, and was for me, although my boarding pass did show my AE number but the change (done by LH at check-in) didn't register in the UA system and the UA segment posted to my UA account. (The short-haul LH segment did go into my AE account.)

So for safety, if the gate or RCC agent cannot make the adjustment to your AE number, you can ask her to remove the UA number completely so you have a blank boarding pass (for status/account) and that can always be sent to AE for later credit.

As for seeing the inventory, I'll check that out for you. Any AC codeshare will not be applicable for E+ of course.