The original reason we wanted to issue warnings was to allow ourselves a chance to change the meaning of parameters, add new parameters, or even remove parameters without causing restore failures. If there are any ERRORs that might limit our flexibility I think we should downgrade those to WARNINGs.
+1
Also, out of a sense of paranoia, it might be good to downgrade some other ERRORs to WARNINGs, like in 0002. I don't think it's quite as important as you seem to think, however. It doesn't make a lot of difference unless the user is running restore with --single-transaction or --exit-on-error, in which case they probably don't want the restore to continue if something unexpected happens. I'm fine having the discussion, though, or we can wait until beta to see what kinds of problems people encounter.
At this point, I feel I've demonstrated the limit of what can be made into WARNINGs, giving us a range of options for now and into the beta. I'll rebase and move the 0002 patch to be in last position so as to tee up 0003-0004 for consideration.
