Backing as far as I understand is of a compatible material the the weld base and can become part of the weld joint. And then removed later if directed. The fixture provided should not make much difference as far as I am concerned because the instance you over penetrate the alum, the chance for suck-back is greatly increased. A weld fixture whether recessed in the back or not shouldn't make a difference. But it got you nervous. Don't forget for D17.1 you get 10% incomplete fusion to the root of the weld leg. Radiograph usually does not find this. And wire brush the living daylights out the back before radiograph so that no aberrations other than the seam show. Confusion on the part the examiner usually results in rejection. Your macro may show the incompleted fusion so don't forget to point out to the examiner the allowance. For instance if your weld leg is .12" you are allowed .012" incomplete fusion to the root.
In the early days of Mil-t-5021d we tacked the ends, and used a third hand to hold steady. All this while the cheapskate supervisor hovered over out shoulders breathing heavily of cigarettes.