I could see "semi-mirror" life evolving step-by-step. There's a few microbes that can eat L-Glucose already (although the enzymes involved seem to be meant for other compounds and just happen to work for it), they just aren't taking advantage of it yet.

That's a far cry from, say, developing reversed DNA and reversed DNA polymerase and all the other transformative and/or structural proteins of the cell.

It's not just about the food/excretions of the hypothetical cell, but about whether its structure and innards are things the rest of the ecology can detect/defend-against/consume.