If you have gunked up plugs, for sure you're burning it. However, there are several ways for oil to get to the plugs.
Bad valve guides are a possibility, especially if the shop that did the head isn't familiar with MB specs. They may have left bad guides in there, or reamed the new ones for clearance, which is wrong. Proper clearance on MB valve guides is basically zero -- if the oiled valve stem will go into the guide by hand, the clearance is fine. If you depress the valve (minus spring) to lift it off the seat and can move it sideways in the guide, the guide has too much clearance. Excess clearance and the oil pours down the engine. It should smoke, though , on startup if this is the case. Also, you will have idle and mixure problems from the air going down the guides -- they can get so bad it's impossible to set the idle mixture properly, with rich running at high rpm. Check via the duty cylce diagnostic.
Another possibility is bad oil rings from being badly overheated -- this anneals the spring on the oil control rings, and the won't scrape the oil down like they are supposed to. Compression will be fine, no leakdown problems since the rest of the rings are OK.
Excess blowby is a given -- but this should give you an oily air filter housing. My TE has a completely dry air filter housing, no oil. So does the 280. However, if the orifice in the other breather tube (the small one between the head and the intake manifold) is too large, you may be sucking oil off the head. Would probably foul 2,3,4,and 5 more than 1& 6, with 3 and 4 the worst. I don't believe th large ventilation hose could get plugged to cause crankcase pressurization, but check that too.
Bad valve guide seals won't give you THAT much oil consumption, I don't think, but they are the cheapest fix.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!