Proof error found in accepted conference paper - what to do?
This is what errata are for.

I had something like this happen to me with one of my early papers.  For that paper, I knew the result was likely to be significant, so I knew I did not want to let the paper stand with an error in it.  It was too late to correct for the conference proceedings, but I could correct the archival version that ended up in the ACM Digital Library, and I did.

Every (reputable) publisher has its own procedure for dealing with errata; you just need to figure out what it is and follow it.  Following this process is likely to be somewhat long and annoying, but if it's important to you to get it right, there's no reason not to do so.