Here's a link for making a special tool to help with fitment of the seal =>
PeachPartsWiki: Replacing Front Crankshaft Seal with "Special Tool"
The flywheel lock is indeed an important part of the job - with the engine removed (yeah I know!) "standard" flywheel locks like BGS 1771 can be used but I've not reached the stage where I've tried to use one of these locks with the engine in the vehicle before.
When I fitted the front seal to my OM617 when it was fitted in the car I ended up using a block up against the front balancer to stop the crank from turning - it wasn't pretty. I guess there's a mega torque requirement for that front crankshaft bolt on the M110 just like there is for the OM617 and M102 (two engine types where I've done this job) - a big effing torque wrench is required.