You can ignore the little light leaks, though it sometimes helps to wear a black shirt when printing and if there are any white walls near the enlarger, drape them with black cloth. The amount of light reflected from the paper is far greater than what comes out of the little cracks in the enlarger. You can tape them up if it makes you happier, just don't block the airflow.

While this isn't quite how it works, think of the focuser as having a little focus screen inside it. Say the mirror is 2" above the paper and the focus screen is 2" from the mirror, that means that the focus screen is the exact same distance from the lens as the paper. If the mirror was at the paper, the focus screen would be too far away! Just do an approx focus, stick the focuser under the enlarger, look into it and then fine-adjust until the grain snaps into focus. If the focuser has an adjustable eyepiece, first set it so that the black line in the focuser is perfectly sharp, then do your enlarger focusing.

Yes, you can load 35mm straight from the canister into a spiral, IF you have set your camera to leave the leader poking out. You can/should even trim the leader (straight across BETWEEN the sprockets, then put 2mm round chamfers on each corner) in the daylight. If the film has been wound all the way back into the canister though, you will need to crack it open to get the film out unless you make or buy a leader-retriever. Don't forget to have scissors in the bag to cut the film from the canister when you reach the end!

While an 50/2.8 EL Nikkor is lovely, you should still be able to focus easily on the grain since it's only 35mm and the grain will be pretty chunky in the grand scheme of things. Consider the case of people who focus 4x5" negatives (grain is 4x smaller for the same print size) with f/5.6 lenses; it's a little harder but not actually particularly difficult.