Most pex can withstand 180 degrees at low pressures. But, I don't think your system will work if you decrease the supply pipe diameter. The cross-section of a 2" pipe is 4x the size of a 1" pipe...you won't be able to pump enough heat around the system. If you used a manifold and ran home runs, maybe, but balancing things might get tougher. See what the pros think, but I think you'll have problems.