RE the offending caliper/potential sticking piston.. I spray brake cleaner directly around the pistons to wash away any rubbish ..I also pump it out to past its usual position when the pads are out to make sure everythings clean.

JUST BE CARE NOT TO POP IT OUT!!

I assume it wasn't a problem before you stripped them down?..

End of the day you have to open the nipple enough to allow fluid out obviously..Im not saying you had it too open that it allowed air in. TBH sounds more like the bleed tube is loose around the nipple.

you could try tapping the caliper/brake line to dislodge any trapped air. Another option is to leave the res top off overnight with the brake lever held back to the bars..I use cable ties.