I'll reply to Q2 only as the economics are for you to decide.
Measure and figure out where your lift anchoring bolts would go and avoid running tubing there. If you can't or won't then if you wet the floor and turn the heat on you can determine where the tubing is fairly easily. A parked car will not block the heat, having insulation and vapour barrier under the slab will greatly "encourage" the heat to travel up to heat the room. In my opinion the heat from radiant is nicer, especially in a high ceiling garage as it heats the area where you are (the lowest 7 ft) and when you are creeping around on the floor if its a while before you get a hoist. Watch out for unintended naps under the vehicles
Have to agree with other posters, slab heating slow/gradual/continuous, not good for fast turn on, a timered/setback thermostat deals with that.