> Can you use the interim state fork (original plus changes, not yet committed into a new immutable) as a "readable" type, the union of mutable and immutable variants?

Not really, not at a language semantics level - although in practice if you applied your current mutation to the original object, did something with the result, and then threw the "partially edited object" away, the JVM's escape analysis might catch that and never fully materialise the "partially edited object". Note that nothing is really mutable, not even your edit operations - you form your combined edit by composing together a bunch of smaller edits, but all those edits are immutable.

Thanks for your patience. I guess the big blocker for the hypothetical "full triangle of readable-mutable-immutable" is that deep read operations would have to create a potentially long lived object for each intermediate step in e.g. a getter chain if you want to avoid special syntax. And then you're back to extreme reliance on escape analysis. And at some point even correctly identified nonescaping allocations start summing up.