I concur that the radiant should be last, in that the temp requirement will be lower. I also concur that the radiant should be separated via a heat exchanger and pressurized. Unfortunately your garage loops have frozen (a byproduct of an open system) and will likely need to be abandoned. Repair is seldom possible without a complete fresh start. The reason atmospheric radiant systems suck, is that the slab is the low point in almost all systems and atmospheric systems WILL take on sediment, scale, oxidation/rust, and other foreign material from the atmosphere which eventually settles in the slab tubing due to low flow velocities and over time plugs up the loops rendering them useless. I see it all the time, and preach hard the importance of closed loop radiant. The "well it works for me" guys are everywhere, but from any standpoint hydronics, they are wrong if they say it's proper.