To the extent any of these solutions involve any challenger being able to “inspect the Plasma chain”, I think they won’t, because Plasma Cash is designed so that users don’t have to (and probably won’t be allowed to) see the entire Plasma chain; they’re only shown their own outputs and the data necessary to prove the validity of those outputs.
But yes, I do think there is a way to do transfers from one Plasma Cash chain to another without touching the parent chain: PoS Plasma Cash with Sharded Validation - #6 by danrobinson.