1. In a city large enough that some uncaring soul would play a Cathar tile, there is a good chance that more than one player has meeples in it, so this would give them an opportunity to recover their meeple(s) sooner than later. Or perhaps in situations where you know the city will never be finished and want your meeple(s) back because you ran out?
2. Yes, one option is to mix the OST into the stack of river tiles. There are no issues with playing this tile at any time because it has one side with grass/farm/field. If you, instead, decide to play the OST after the lake, the rules are not entirely clear if the placement of the lake tile is considered to be a person's turn, or whether it occurs in between turns. Other options for the OST include leaving it out of the game entirely (as per the Count rules) or mixing it in with the other land tiles (not an official recommendation).
Thx Scott!! for 1, i din't thought about that but this is a real nice point!!
#2, it's not clear "in the rules" may RGG or Hig can offer us the "official ruling??"
thx again Scott!!
Deatheux(Death métal rulz!!!)