I am a newbie to MBZ and I too did this repair and had the same exact problem, where after loosening the 3 pivot bolts, and almost removing the tensioner bolt, the A/C compressor would pivot a little but not enough to get the new belt on. After an hour of inspection, I thought it might be the Oil Lines bolted to the compressor so I just removed the retaining bolt (9mm I think) so the compressor was free of those. Sadly it wasn't the issue. Upon further inspection, I suspect its actually the compressor lines themselves that are restricting the movement the A/C compressor. They are routed quite complicated and are bolted all over the place to hold them steady. You don't want to remove those lines from the compressor and unbolting them looked like a chore (perhaps its not but it looked like it on my '83 300D.) So there is no easy answer to get the full play in the A/C compressor. However there is a TRICK to get the belt on - at least it worked for me. Its the same way I use to re-attach the chain on my no geared bicycle (1974?)

With the new belt around the AC compressor pulley, put as much belt on the engine pulley (2nd channel counting from the front) from the BOTTOM and keep working your way up pushing the belt on more and more while pulling the A/C compressor toward the engine pulley. At some point, you will max out pivoting the compressor and stretching the belt. If at this point, the belt will stay on but is not all the way on, you should be good to go. Simply crank the engine over once or twice. The belt should feed itself onto the engine pulley. Done.