Do the reinforcements count towards the 50% threshold that need to reach the North edge?
No. And do remember that Elementals can airdrop on their own accord - they don't necessarily need a ride.
Rob what's your thoughts on the terrain?
If you want drifting units to land on and then collapse buildings, you want Light Urban, not heavy Urban. Heavy Urban will involve more Hardened and Heavy buildings with enough CF to stay upright.
Initial thoughts are that this looks about right for a B-rated formation, even if there's more Star League Mechs and fewer IICs than I would have expected. IMO we should be looking to delete as many light units as possible on Turns 1-3, before they can land and group up. A) That hurts their ability to move towards the north edge, and B) Clan Lights pack a lot of firepower on a fragile chassis, so deleting them out front maximally limits their outgoing firepower most efficiently. But we'll actually have to do shot-calling and not be lazy the way we normally do.
Big downside is that we aren't going to be able to use Artillery in a city, while we could if we were away from Urban terrain. Defensive mission, remember. These are, for the duration of the contract, OUR people we're defending, so we can't drop Long Toms on their houses.
Reinforcements would be limited to Jazz and Lancer, because everyone else is in Training, and Jazz has a Solo the day before, so if we have a reinforcement, it's likely Lancer Lance.