> For this database has to have nested transactions, where COMMITs do propagate up one level and not to the actual database,

Correct, but nested transaction support doesn't seem that much of a reach if you're an MVCC-style system anyway (although you might have to factor out things like row watermarks to lookaside tables if you want to let them be branchy instead of XID being a write lock.)

You could version the index B-tree nodes too.

  > but nested transaction support doesn't seem that much of a reach if you're an MVCC-style system anyway

Also, do not forget about double COMMIT, intentional or not.