Durability of aircraft to MG (< 8mm) guns

As for aluminum skins deflecting and dissipating energy from MG rounds, yes and no.

Aluminum is a very soft metal and bullets have little to not trouble penetrating multiple layers of it. Sure you can have a bullet impact at a very extreme angle or at the end of its effective range, but for the most part rounds keep on trucking. A guy I know went out to the bush to test out a new (new to him, but old) .30-30 rifle. This is pretty much the weakest .30 cal rifle out there. He accidentally (negligently) put a round through the side of his mini-van and it went clear out the other side. It went through the outer panel, through one of the support beam leading to the roof, through the plastic casing, through the entire second row passenger seat (metal frame, 4 feet of foam, and then through the other side's plastic casing, support frame, and outer panel.

Bullets do a lot of strange things, but against soft or thin metals they usually go straight through and only vary their trajectory slightly. Hence He-111's coming back intact, but with mostly dead crewmembers.

In your typical WWII airframe the only things that are going to stop them are armour plates, metal cylinders, the engine, or other hard metal fixtures that are not part of the actual airframe and shell.