X10, on everything said above. If you don't get this figured out real quick, you'll end up with a huge money pit. If you get enough water/coolant into a cylinder, it is possible to hydorlock the engine. Right now, white smoke (probably coolant) is passing through the engine and out the exhaust, which can contaminate the O2 Sensors and the Catalytic Converters. If your fuel economy sucks, you may already fouled the O2's.
Pressure check the coolant system. If you're losing pressure, you must determine where it's going. Pull the top set of plugs and crank the engine to see if you get any water out of any cylinder(s). Be sure to pull the ASD relay. Are the low cylinders all on the same cylinder bank? Do you have the actual readings for each cylinder? Another way is to pressurize each cylinder and see if you're getting bubbles into the cooling system at the radiator. There are other ways to check this also, but these will narrow it down a bit. Also, I wouldn't run the engine any more, until you figure it out and get it repaired. Some of the main culprits here could be, cracked head, cracked block, or blown head gasket. Do you know if it has ever been overheated? This is very unusual for a vehicle with so few miles, unless the vehicle has been neglected and abused.
As far as the oil burning and low compression goes, a cylinder leakdown test would give you a clue as to whether the valves are bad and/or the rings are bad. You can also use this process, as mentioned above, to check for bubles in the cooling system. Problems here could be piston rings, leaking valves, leaking valve seals, etc. Good luck and let us know what you find.
06 GCO, Dk Khaki, QDII, Flshing Hi 3rd Bk Lt, MDS "ON" Lt, Airtab Vortex Gens, Debadged, Fumoto Drain Valve, BT Catch Can, Chopped EGR Tube, SRT8 CAI, K&N Filter, Sonnax Trans Line Press Booster, SC Tuner W/Adv Trans Tune App, DS Tuner, Cust tubular hdrs w/3 in Magnaflow/Xlerator AP Exh Sys, Bilsteins, SRT Springs/Sways, Cust Eng Cvr, SRT8 frt/rear bumpers, FRI Sidewinder Cam/6.1 valvetrain, SLP 25% UD Pulley, BBK true 90mm T-Body, SOS Ign Coils w/.060 spark plug gaps, SRT Reps with 265/45&295/40X20's.