I couldn't tighten the endcap on all the way by hand. If I somehow
managed to get it on all the way, would it be water (and, more
importantly, aluminum) tight? How would I tighten it that far, when
the endcap is about 5 inches across?

I don't see why (or how) it wouldn't. Pipe threads are made to
seal...

Thread friction in that size is a lot. Consider a turn of
teflon tape or teflon thread sealer. Not so much to act
as a sealer in and of itself, but to lubricate the threads
enough to really torque the cap down to get a good seal.
For a cap that size, a vice and a serious pipe wrench is in
order. I would be tempted to say that it is not possible
to overtighten it with hand tools...

--Glenn Lyford