So basically what you're saying is, use the salty shrimp in my top off water until i get my desired gh, then just go back to RO to top off? Then, match the gh of my water, with the new water, when it's time for water changes?
What I'm doing right now, let me know if I'm wrong, but I'm adding the mineral straight into my RO/DI water, mixing it up and adding it straight in as top off water.
What DaveKS said is right.
Water evaporates, minerals don't... so if you top off with water that has minerals in it, you are only increasing GH/KH (if there's any KH in water) over time... which could make the water very hard.
If the GH and KH is where you want it in the tank, then you would only top off with RO water and do water changes with water that's been remineralized to what you want in tank, or just below that (in case in tank parameters have risen).
Since your GH is already low, it's fine to use remineralized water to top up for now... it will slowly raise your parameters so it's less of a shock to the shrimp.
There's a chance that the offspring are currently "okay" with low GH parameters but the adults need more minerals... and they might be dying due to lack of minerals. Or there could be a different issue... hard to say for sure... but algae should not cause deaths unless the tank is overrun with algae and is somehow causing harm to the tank?