If you understand how an air conditioner works, then you understand how a heat pump works.
Many radiant floor heating systems have used not boilers, but water heaters, and VERY successfully. Water heaters are out of the box designed for lower operating temperatures and handle low water return temperatures just dandy.
The original Levittown designs in New York, Pennsylvania, and where ever else they were buit used slab radiant heating systems and were extremely successful. Unfortunately, though, many of these systems have now been abandoned because of leaking copper pipes (and I wonder how much, if any, insulation was used under and around the slabs?). Many Levittown houses have been converted to either forced hot water baseboard (not bad in and of itself), hydronic (water/air exchange) systems, or water has been totally abandoned for forced air.
New systems using PEX as the heating tubes are promising to eliminate slab leaks (unless someone rams a nail through a tube), and the technology has been in use in Europe since just after WW II where many original PEX systems are still in operation.
As for hardwood over in-floor radiant, it can be done successfully. However, I feel it's a much better choice to go with engineered laminates when installing "wood" over radiant.