If you were Aegon the Conqueror, would you have included House Lefford of the Golden Tooth into the Riverlands under House Tully’s Lord Paramouncy or kept them as part of the Westerlands? (P.S. I’m asking Attewell, SLAL and the Good Queen, having huge respect for all y’all’s differing POVs, TY)

Well, it sort of depends on how, as King, you view the loyalty of the Westerlands and the Riverlands. After all, the whole point of Aegon’s Conquest was to assert the unity of Westeros and establish a scenario where wars between kingdoms don’t happen, and given that the only purpose of the Golden Tooth is to ensure that the Westerlands can’t be invaded by the Riverlands but can invade with impunity, the King’s rule should make the Golden Tooth irrelevant. 

So the only scenario where I would see Aegon giving the Golden Tooth to the Riverlands is a scenario in which the new king sees the Riverlands as more loyal than the Westerlands, and needs to forestall any future rebellion which might involve an invasion of the Riverlands. 

Indeed, if it wasn’t for Aegon’s dracocracy leading him to make minimal changes to pre-existing conditions, it wouldn’t be a terrible idea to annex the fortress to the crown to prevent either kingdom from invading the other without openly rebelling. 

If more realms than just the Reach and the Westerlands had fought at the Field of Fire do you think it might have gone differently, that they might have been successful in stopping the Targaryen invasion?

Hell no. That just ups the body count. If you’re trying to defeat Aegon’s Conquest you don’t do it with large-scale field battles, because that’s just a target for dragonfire. 

You do it with Dornish tactics. Split up your armies, abandon most of your castles (Casterly Rock being a noted exception) hide in wildernesses and inside populations, ambush his armies and then vanish rather than try to hold territory, stretch his forces out and then strike where the dragons are not. Wear him down and pick off Targaryens where you can.