To address most of those problems I'd suggest limiting it to one house per knight with a minimum rent of 10 gold. I wouldn't put a restriction on how many houses an adventurer could lease, but rather make it hard to get them and unlikely they'll get their hands on all of them.
Also when I came up with the idea I thought it might be restricted to a new third adventurer rank called "Yeomen".
The possibilities for adventurer to adventurer interactions are almost endless, especially if an adventurer can rent his house out to other adventurers.
There's also quite a bit of motivation for adventurer to noble interactions. For example an adventurer could potentially try and bribe a knight into evicting an adventurer so he can take over his house.
Plus this can create roleplay potential for region lords and their knights. Maybe the local lord doesn't like having all these freeloading adventurers in his region, so he forbids his knights to sell to them and orders all existing peasants to vacate at once.
Also it could be that there's a chance of hurting region moral if an adventurer is evicted, the idea being that the peasants liked him/her and the fact that they kept the beasts away.
