I'm also getting the cannot find dbxquery error after upgrading to 6.3. Wasn't an issue pre 6.3.
I have, in this particular deployment, 1 search head, 2 indexers and a master similar to you but without heavy forwarders. Prior to 6.3 I didn't need the app on the indexers. It seems now that in 6.3 more things are published in the distributed search bundle than before? It's like more search functions have now been moved to the indexing tier (probably to better enable parallel processing pipelines?)
I haven't gotten to forcing the app on my indexers as a workaround like you've done and it seems that won't do me much good anyway.
The way I understand it is the recommended way to use dbconnect is on an ad-hoc searcher, not part of your cluster and not a distributed searcher of your indexers but as a heavy forwarder instead. I don't see many people do this because how are your lookups going to work?
For your solution, I'd recommend configuring dbconnect solely on your heavy forwarders and even have a third instance running the RPC to avoid duplicate queries.
For now, I've gone back to dbconnect v1 where possible on 6.3 instances which seems to work just fine.
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