OK, I think I've come up with a solution to Option A.

As mentioned, Base64 includes 3 characters that are NOT valid in element IDs: '+', '' and '=' and we have only two available characters to use to replace them: '-' and '_'.

However, the '=' only appears on the END of the encoding, as padding characters to even the bit count.

Never mind. I'm an idiot.

When I said the correct ID was being passed I was mistaken. I had the ID of the row in the subgrid that triggered the reload, NOT the ID of the row in the parent grid.