Awesome work.
I have the feeling that the aliens collision box is shifted to the left or that they don't cover the right area of the alien.
That is entirely possible. It may well be that the sprite coordinates are not placed exactly where they should be. Early hardware had known nuances with some (eg. 1 or 2) sprites being offset a few pixels with respect to the other sprites. In fact, I'm currently working on Frogger (same hardware) and did have to tweak the sprite routines that I lifted from Galaxian; Frogger has the advantage in that the background can be used as a reference, though of course the background is also subject to its own nuances. I'll finish up on Frogger and revisit Galaxian to see if there are any changes I should back-port.
I'm also thinking it may be possible to implement a non-tate mode for Galaxian as well!