For Nylog, you need to use the one that is for your intended refrigerant. The green o-rings are fine for 134a or 12 (and others).

10/liter is pretty normal for AC flush. Yes, you can fill the parts with fluid and use air pressure to move it around. Sitting flush agent doesn't do nearly as good of a job as moving agent.

Pulling a vacuum overnight won't help too much. You should get to where you need to be within an hour or two. At these sorts of pressures, your manifold set is pretty much useless. You can hardly tell the difference between 28.8 and 29.8" vacuum. But that's a range of 25,000 microns on an electronic gauge and is very easily seen.

I do not recommend pulling a vacuum and letting it sit overnight to check for leaks. This is because the procedure will ruin your dryer if you do have a leak, and you'll need a new one for the next check.