Would a precharged JO crystal fit in place of the original cap?
I think my Dup held its saves for a few weeks, however, some of the more important ones (e.g. Ys I&II) are backed up on a Tennokoe bank card.
Regarding coin cell batteries: Big brand name batteries last longer than the cheap ten-packs. Since my better half uses a lot of CR 2032 cells for her cosplay lighting, I had the opportunity to try and compare a few brands. I had a cheap CR 2032 cell in my Sigma BC 5.12 bicycle computer, it did not even last two weeks. Compare this to a more expensive one that now last two years of daily use (display is active for showing time, even when not in use, but no backlight whatsoever available) and is still going strong.