Played a game as Arpad. Felt pretty weak but I liked the ideas and the mod worked as advertised. I was able to found a few cities that got free walls and yields from barb camps that I'd cleared, but razing a city just for those bonuses doesn't seem worth it.
I think it would be great if the bonus for founding a city near a razed city specifically would be immense---maybe a percentage production/culture boost + free buildings. Make it so that you're actually incentivized to raze and refound a city, even if the captured city had some infrastructure in it already.

It works like Cree's ability. The first non-owned tile that a military unit enters is claimed by your city. So make sure to found your capital before moving your starting warrior to get a free tile from turn 1.

I'm guessing it means the city's borders expand by 1 when training a military unit.

I dont know what exactly you mean by military units claim one title. Like they can culture bomb?