Can't anymore, could before.
Mennelon
Legacy of Serrance tooltip in Hero Selection and Faction Editor do not mention also giving the Unique Improvement: Bazaar or Unique Equipment: Warhorse items Legacy of Serrane tooltip in Kingdom Report does mention both of these items, but states that Warhorses increase weight capacity and initiative - they do not increase initiative (tooltip error or item bug?).
I think probably the best way for governors to be governors would be to let them gain 1xp/turn in a city, and +10xp upon improvement completion while in a city. You still won't level as fast as a combat hero (not even close) but you'll be providing full benefits to a city and at least gain a couple of levels. Late game you can probably afford to convert mana or gold to xp (destiny's insight or quest maps) for your governors, but early/mid that's not goin
Only affects the building on that tile, not the whole planet.
Thing I've started doing when that happens is keeping a few quick ships on hand (custom colony or troop ships) to ferry some population over there and get it paying for itself quick without having to worry about pop. growth. ed. Also has the bonus of preventing a quick sneak attack with a single transport from taking the planet back immediately
New player myself, but can think of one other reason why it may not have flipped. That race might have had the loyalty trait (Yor get 100 loyalty, others get some less, and it might be taken as a random bonus) which makes it harder if not impossible to flip them culturally.