Have AS issue the ticket, phone AA and have them put your Aadvantage numbers on the record, which should allow you to manage your booking using AA's website. You probably would also want to get AA's record locator, which will likely be different from AS's.
As for lounge access, just have your Plat cards in your possession at all airports, and you shouldn't have any difficulties.
Remember AS can only issue tickets on their own, or one other partner's equipment, so if you want to go to someplace in Europe not served by AA metal, you'd have to pay the difference on your own from the closest AA landing point. Using AA miles, you could use any number of partners on the same itinerary. If you're short of AA miles, you could probably buy them more cheaply than paying for a ticket. AA often has "specials" on miles purchase (20% bonuses, sometimes 30%) and since February is a slow time, getting award space usually isn't a problem, so you could wait for a sale.
[Moderator hat on] This really isn't a Oneworld forum topic; I'm going to move it to the AA board for further discussion. [/hat]