Thanks, Nick and Stefano for the info.
As for the question about two apps needing two different indexing stragegies, it was a question of curiocity and I don't have a use case yet. But it is conceivable that one app only uses node identities while the other uses the accounts. If my understanding is correct, the former will use PUBLIC_KEY and the latter EXTERNAL_ID.
In that regard, I did a test with both in the conf like this
indexingStrategies: ["PUBLIC_KEY", "EXTERNAL_ID"]
and it seemed to work without any complaint. So not sure which strategy was actually used. BTW, the doc says only "And choose a single indexing strategy, ...".
Also, per Stefano's reindexing, how does it work? How can one request another indexing type on the fly?