Should I reorder RADB,ARIN to be ARIN,RADB and similar for RADB,RIPE and RADB,APNIC,ARIN? Tentative: Yes.
Should I drop RIPE-NONAUTH choices? Tentative: If nobody is using it by the end of this, I'll just remove it.
Does anyone know if AFRINIC, APNIC, and LACNIC are authenticated?
Does anyone know if people care about LEVEL3's IRR? Tentative: If nobody is using it by the end of this, I'll just remove it.
How do I set the source per participant? I think what we want is the following, for each participant:

I use RADB's web interface to view the specified as-set. If the "source" of that object is an RIR registry, e.g. ARIN, I set the source in IXP Manager to that RIR source. If the source is RADB, I set the source to $RIR,RADB where $RIR is the RIR that issued the participant's ASN. For example, if someone is in the ARIN region, ARIN. If someone is in the RIPE region, RIPE. The idea is to make it easy for them to move to the RIR registry in the future.

I manually trigger the AS and prefix update and make sure I get results.

If the source is just $RIR, I then change this to $RIR,RADB. I manually trigger the AS and prefix update. If that second update changed nothing, I remove RADB. If that update did change something, then I eyeball it and make a decision. I would be looking to detect the scenario where e.g. the as-set is in the ARIN region but references customer ASes/as-sets that are in RADB; in such a case, I need to allow RADB. In contrast, if the records in RADB appear to be outdated and someone has moved to ARIN, I would want to remove RADB.

If that becomes too tedious, a fallback option is to set everyone to something like ARIN,RIPE,RADB or ARIN,RIPE,LACNIC,APNIC,AFRINIC,RADB. (The order of the RIRs is negotiable. I don't really know much about non-ARIN IRR.)