Is it possible for a search head cluster to search multiple index clusters?
A Solo Searchhead can do so, but I'm not sure with the Searchhead Cluster.
Yes you can.
Here is the doc that shows the configs to do so.
Yes, you can have single SH or SH-cluster searching multiple indexers and indexer clusters as well as single indexer/indexer cluster being searched by multiple SHs. Or any combination of those options.
Yes it should be able to. You can configure multiple multisite indexers to be searchable to a SH cluster.
###If it help, kindly consider accepting as an answer/upvote###