Hornady, and Tulammo are currently available. Marstar also said that they have the ability to bring in cheap surplus 5.45x39.
Nothing firm on pricing, but should be below $1000
You are correct. The 5.56 and 300blk only need a barrel swap, but the 5.45 will also need a bolt face and firing pin swap. (takes approx. 5min)
No higher capacity mags at this time, but are planned in the future.
Firing corrosive shouldn't be a problem, as the barrel is nitrocarburized.