how do I turn the flywheel on my D2 to access flywheel clutch bearing?

The sticky lube used in the idler and roller is a soft semi liquid that is pumped out of a low pressure can. Your zerk gun is a high pressure unit and may push the seals out of the rollers and idlers. Anything else on the tractor that has a zerk fitting you can use it on. So I wouldn't go and buy a bigger one. As far as the clutch bearing, set your compression release, engage the pony pinion and clutch, open the petcocks on the pony and turn it over by hand. This will turn the flywheel for you.
Unless your hydraulic pump is on the back in which case you would set your compression release and put a crank in the front and turn it by hand.

The waterpump fan hub has a relief fitting on the upper right side and a grease fitting below it on the right side of the hub. you can apply grease until it comes out the relief valve.

If you don't already have an air operated grease gun, Lincoln and probably others make battery operated guns these days which would be a lot less cumbersome than the air operated ones. They would be dangerously high pressure for track rollers etc unless you have a coupling for the buttonhead fittings with a relief valve in it.

The battery ones are the ticket for greasing by yourself. Sort of like high speed internet access, once you ever try it, it's hard to go back to the old way.

The waterpump fan hub has a relief fitting on the upper right side and a grease fitting below it on the right side of the hub. you can apply grease until it comes out the relief valve.

Check to make sure you have a relief valve first. A lot of the Cats around here have been Californicated; the relief valve has been replaced with another zerk. You will eventually damage your seal without a relief. One pump is all it needs.

I just greased the fan hub on my 7U and it took at least twelve pumps before it came out the relieve valve.