Complete Failover Cluster Failure SQL Server 2012
If you evict nodes you'll end up formatting the nodes and rebuilding them.  Once a node has been evicted from the cluster you won't be able to uninstall the SQL Server instance from the node.  You can try manually removing all the registry keys and files, but honestly it's probably faster to reinstall Windows and try getting it back into the cluster.

Has Microsoft not been able to get you towards any resolution on this?

Either way you're looking at a cluster rebuild basically from scratch here with 21 SQL installs, which I'm sure as you know takes forever.  There are some ways to make this take less take (building a new two node cluster, installing everything there, then doing a controlled migration between clusters, use DNS to redirect connections to the new names so applications don't need to change, etc.

If this was my environment I'd burn it down and bring it back fresh, obviously with as little downtime as possible.

--sales pitch--
It might be a good idea to bring someone in that has a lot of clustering experience (me, someone else, either way) to help with the rebuild to make sure that everything is setup exactly as it should be and to help take the pressure off your internal team (who probably hasn't been getting a lot of sleep the last two weeks).  If you've got a lot of clustering experience in house then this isn't needed of course.  I'm just worried because something went horribly wrong doing something which souldn't have been that big of a deal to do.
--end sales pitch--

Disclaimer: I'm a consultant.