Searching for 30 years, I have been trying to figure out the "right" way to finish wood. A subject near and dear to my heart. I too applaud your experimentation, but would like to make a few points:
1.The fir your door is constructed of is probably a bit different than the scrap wood you're experimenting on....the door manufacturers use a grade of wood that's probably a lot more uniform in moisture and pitch content, although this is sadly changing as decent wood becomes scarcer.
2. With wood finishing, a quality product specifically designed for the application (as Steve so often points out when discussing paints) is essential. (I've been having some really great results from General Finishes products sold at WoodworkersSupply. Go to http://generalfinishes.com )But then again, they are working well for my applications and I haven't tried their mahogany stain on fir.
3. When you finally get the effect you want, someone will buy the company that makes the product (Like Watco) and change the formula.
4. Go buy the book "Understanding Wood Finishing" by Bob Flexner. You'll understand then why Minwax is great for some applications and disappointing for others. It's maddening trying to get a nice dark even finish like Minwax does on walnut or mahogany on pine or fir. Flexner explains why and gives great alternatives and the book is richly illustrated. I work a great deal with Western Red Alder and avoid the splotchiness by using dye vs pigment stains.
5. The reason I asked if you had a spray rig is that you can spray the Minwax PolyShades, which is a tinted polyurethane (and intended to be brushed) to get progressively darker coloration. The spraying makes it a lot easier to shade and even the effect. But you'd be much better off getting the color you want with a stain (dye stains work better on the fir than pigment stains for darkening and evenness. A much better route to go than with the PolyShades and that a lot of the production cabinet shops use is the Dayco Blondit products (found in better paint stores). It is a tinted lacquer that lets you do the same thing as spraying PolyShades with all the forgiveness of lacquer. Actually a tinted synthetic lacquer, I think.
6. I really like the MinWax Polycrylic for a top coat. It dries really fast like a lacquer, is water based and doesn't yellow like a lacquer.
Sorry to be so long winded, but you can tell this is a meaningful subject for me. Good Luck, Kizmet