None of these wood species are inherently weather resistant. They all depend on the paint, stain or sealer finish for weather protection. So the answer to your question is found in how you propose to finish the door. Maple is a light and very hard wood, It will take stain, sealer or paint equally well. Douglas Fir is considered a soft wood, but one with great structural ratings, when used in framing lumber. It is a light wood, but not one that is particularly attractive when stained, IMO. Mahogany is a dark wood, and probably the most naturally weather resistant of the three. It becomes even darker when stained or finished with a clear sealant. Painting Mahogany makes little sense. Hope this helps. Good luck with your project. So much depends on whether you will paint or stain the door, or you think you will leave it weather naturally. The latter is probably a mistake.

A great tip I learned when filling cracks etc. in wood is to sacrifice a small amount of wood from the item, make sawdust from it and mix the sawdust with the wood filler, since you are going to paint the door and there probably isn't anywhere you could trim a sliver off without affecting the seal of the door, simply match the type of wood your door is made of and make some sawdust from it and mix with your filler.

Door skin isn't normaly made in 1/8th increments,more like 32nds so yours is likly a common thickness. A belt sander would remove material very fast BUT, with presision slightly better than an axe in the hands of most woodworkers. The surface must not have hills and valleys when skin is glued. I know you can't buy specialized tools for this but that might be what it requires. A body file used to level bondo just might be the ticket. If you mentioned belt sander because you already have one,if you are carful you may be able to remove 75% then go to hand sanding. The paper should be backed up with blocks as you move through the grits,final grit the coarsest that doesn't leave a bunch of wiskers. When you think the surface is level and 98% of skin is off,lightly mist the surface with quick drying paint like krylon or laquer in a contrasting color. Don't coat the surface,just a mist that looks like overspray. Use final grit backed with a 2x4 51/2" or 9" long to sand the paint mist off to reveal low areas. 75%+ of surface paint free is glue ready. That's only one method but hopfuly gives you ideas of your own using tools on hand or that can be borrowed.

Thanks for responding. I did try several easier things before stripping. And, before stripping I cleaned with mineral spirits, degreaser, denatured alcohol, and TSP in hot water with microfiber. After TSP escpecially, the surface was extremely clean. I don't think I'll have any problems with silicone, but my game plan includes washcoat with shellac in any case. I considered hiring a pro, but the fine light-colored scratches in the middle picture mostly couldn't be seen across the top surface of the finish, so they were defects below the finish. So I am pretty sure a pro had already sprayed some toning lacquer over these scratches, reducing their appearance but not eliminating them. Just wondering if anyone has experience getting stain color out of scratches? Or if the color in these scratches will even matter if I use dye stain? I do have a game plan, and I have experience stripping and refinishing, but never on mahogany veneers like this.