Utility door experiment

I'm in San Francisco and recently stripped, sanded (to 320 grit) and refinished an old fir door for my front door. I stained it with Minwax Wood Finish cherry, then put three coats of Sikkens Cetol Marin Gloss polyurethane. The door faces N20W, is covered by a 4' square overhang, and is on the non-weather side of the house (winds always come from the back of the house) so gets very little water. I did this with very little research; just figured that boat polyurethane would be best.

Now I am putting a 12"x15" utility door on the N70E facing side of the house. It will only get morning sun as the neighbor's attached house shades it after about 10am. I am not too partial to the plastic-coated look of the polyurethane, and am wondering if I could using an oil or stain on the door instead. Unfortunately I barreled full steam ahead, intending to copy the front door, so I already stained it with the Minwax.

From what I've read on this site, looks like Defy Extreme stain would be the top choice available to me. So, a few questions about compatibility and application:

Would I need to strip/wash/clean off the Minwax or could I apply Defy right over it? It is the color I want (matches the front door).

The wood is old 1" (true) fir shelving scavenged from a 50+ year old school. I assume it won't have the problems of new wood, but will the fine grit sanding cause absorption problems? Should I resand with 120?

The UV exposure is less, but not non-existent, so could I go with a tranparent or clear if I can keep the Minwax? Or is that still too much sun? If I should opt for UV protection, what should I do and/or what stain color should I choose to minimize color change?

I will seal all 6 sides; although it is not on the 'weather' side, it has no overhang and will get wet. The application is wet on wet, so do I pick a side to 'sacrifice' (the inside) and do all 6 sides at once, leaving it to sit on the 'sacrificed' side (up on two mitered boards) or do I do the outside and 4 edges first and come back and do the inside later?

I intend to refinish yearly but leave it hung when I do it. Thoughts?

Thanks