Is the axle clean? Could be a small amount of dirt in the threads causing bother. Failing that, take the pinch bolt out and use a screwdriver to gently open the collar out a bit, that may make it turn. A bit of oil or penetrant would help as well.
Edd
9 speed chains will run with great difficulty (or not at all) on a 7 speed cassette, but a 7 speed chain will run on a 9speed chainset with just a bit of side play :lol:
New seals wont come from magura, they dont like you faffing with sealing for obvious reasons. RB seals wont fit. So either fetch the old seal out, and go and see if somewhere like RS components does them, or find a knackered lever to cannibalise.
If the elbow (bit that hits the frame) is silver rather than black, you'll need to put a 13mm spanner on it to move it, and it might take some force to move.