The other extensibility that seems quite clear and uncontroversial to me is 0006.
It simply shifts the decision on whether tuple inserts should invoke inserts to the related indices to the table am level. It doesn't change the current heap insert behavior so it's safe for the existing heap access method. But new table access methods could redefine this (only for tables created with these am's) and make index inserts independently of ExecInsertIndexTuples inside their own implementations of tuple_insert/tuple_multi_insert methods.
I'd propose changing the comment:
1405 * This function sets `*insert_indexes` to true if expects caller to return 1406 * the relevant index tuples. If `*insert_indexes` is set to false, then 1407 * this function cares about indexes itself.
in the following way
Tableam implementation of tuple_insert should set `*insert_indexes` to true
if it expects the caller to insert the relevant index tuples (as in heap
implementation). It should set `*insert_indexes` to false if it cares
about index inserts itself and doesn't want the caller to do index inserts.
Maybe, a commit message is also better to reformulate to describe better who should do what.
I think, with rebase and correction in the comments/commit message patch 0006 is ready to be committed.