> 3) Use reltoastidxid if != InvalidOid and manually build the list (using > RelationGetIndexList) otherwise?
Do we actually need reltoastidxid at all? I always thought having that field was a case of premature optimization. There might be some case for keeping it to avoid breaking any client-side code that might be looking at it ... but if you're proposing changing the field contents anyway, that argument goes right out the window.
Here is an interesting idea. Could there be some performance impact if we remove this field and replace it by RelationGetIndexList to fetch the list of indexes that need to be inserted?