This doesn’t sound self custodial to me. You don’t even control the keys.

“Spark operators” help facilitate transfer (by signing the transaction) so that means the user is one of the keys. What happens if the spark operators decide not to sign the transaction? Who maintains the spark ledger? Isn’t that a trust system? If it’s not in a blockchain?

It's so easy to go to docs and click on search button and make any question on

Welcome to Spark - Spark

The search works like an IA. I'm sure all doubts can be easily answered there.