Peter,
Yes, your analysis is correct. That error is indeed due to the SolMan SLD not being capable of being a target for forwarding.
The solution in your case is to choose your PI SLD (probably the PRD one) as your "central" SLD, configure your SolMan PRD SLD to forward to it, and configure your SolMan PRD LMDB to receive its syncs from the PI SLD instead of its local SolMan SLD. Then, gradually, over time, configure all your data suppliers to send their information to the PI SLD instead of the SolMan SLD. Once they are all switched over, you can turn off the forwarding and deactivate SLD on SolMan.
Whether or not to have your various QA SLDs forward to your PRD SLD is a different landscape design question. I only use a single (standalone) SLD in my landscape, so everything points to the single source of truth, with one exception. I have a "DEV" SLD on my DEV SolMan, so my DEV Solman isn't receiving LMDB data from the PRD SLD... instead it's pretty much an isolated system, just for testing changes to SolMan. Anyway, there are a number of descriptions of best practices for reliable multi-SLD landscapes linked from the More on System Landscape Directory blog by Wolf Hengevoss that you may find interesting to peruse.
Cheers,
Matt