It's not at all unreasonable to ask. That doesn't mean that anyone will have the answer you are looking for....
I really don't think its a stupid thing to ask for a road map or infographic, as I believe an airplane would carry key components.
There isn't a textbook on sim aircraft design. There are SDK's available, but they are targeted squarely at people who already know what they are doing. They can be pretty tough going for amateurs, myself included.
The list of tools and tasks you suggested - probably not too far wrong as far as it goes, although I suspect there would be a few additional tools to add.
OK, so I went back and re-read your post. You're not too concerned about the time component. That's certainly good, but I do want to offer some perspective here.
If you're working 9 to 5, Monday to Friday, with 4 weeks vacation, and 10 stats, you're working 1610 hours per year, give or take a few summer Friday afternoons.
A really high quality add-on aircraft will have absorbed several thousand hours of work - something of PMDG complexity, probably on the far side of 10,000 hours. (Will everything they do take that long? No, they'll re-cycle a bunch of their code.)
You really are looking at a very long time between now and release, if you are planning on learning it all as you go.
To be honest, I think a more practical way for you to get the aircraft you are looking for is to get good enough at something that somebody wants you on their team. My suggestions would be either C gauges or aircraft flight dynamics.
If you still want to do it all yourself, then start like this:
Open up the folder for one of the stock aircraft and have a look through the contents. This is really your roadmap - you're going to have to come up with the equivalent of everything you see inside.
Read the SDK to learn what everything does and how to go about making changes. For example, for a given aircraft, if you use a different model file, what changes are you going to have to make in aircraft.cfg so that the thing sits properly in the ground and not 3 feet above or below the ground. That kind of thing.
Once you understand how all the stuff comes together to produce a simulated aircraft, you are probably in a better position to start making modifications. Maybe you're already there, but it never ceases to amaze me just how many people, who take this whole thing way too seriously, have no idea how any of this stuff actually makes their simulated aircraft work.
Now pick something you want to modify and look, in detail, about how to go about doing that. Stick with it until you're at least passably good.
Something you would likely not know about this particular forum is that there is a very real expectation that you have tried to accomplish something on your own before you ask for help. That goes for specifics, and, as you have discovered, generalities.
That said, you will likely get a better reception if you post a 3D model, or part thereof, and ask - how come I can't get this to do X Y or Z? Or post a block of xml gauge code and ask why it isn't showing on the panel.
It's just the culture of the place and not something that you or I will change over night.
I will close by re-iterating what has been said before. Start small. It goes a long way towards boosting your own interest in continuing if someone asks, of something you've done - "Hey, can I get a copy of that?" You need to be able to finish something for that to happen.