Interlux Perfection (formerly Interthane Plus) is very easy to use for a DIY application as it is designed for roll/tip or spray. My deck/coackroof was sprayed with Interthane Plus 5 yrs ago and has held up well. If just doing part then think about the non skid areas. Much like flat paint vs gloss is very forgiving. You can then tape off that section and do the rest later.
I have been old Awlgrip is very difficult to touch up but have had no experience with that. OTOH I have used Interlux one part Brightsides to touch up any areas where I do repairs on my deck with outstanding results. Also you can choose the same colour in Brightsides and Perfection (I use Off-White).
The one part levels better but the two part lasts longer. I would use 2 part. Around the toerail you could probably put the paint right up to toerail and then apply a bead of 4200 or something to cover the edge of the paint - much like 1/4 round when laying flooring at home or silicon around a tub edge when laying linoleum in a bathroom. Everything else should be removed.
As for one part now - two part later. Is not a problem if you do the prep work properly. Interlux two part primer is designed to protect "unknown" paint from the harsh two part paint being applied over top. As our boat had a failing unknown paint on the deck I sanded, sanded and sanded some more and then primer coat and two part paint. Have experienced no problems in 5 years.
The painting is the easy part - it is the fixture removal and prep work that takes the time. Also - when removing windows you will discover the joys of butyl rubber. When people tells you it does not harden they are correct. It also gets into everything and you spend half your day getting trace amounts off the deck that you wish to paint!