Beginner Seeking Answers to Several Questions

#9 it depends on source model's license, but in most cases an asset that was build upon someone else's work cannot be sold on stock marketplace.

1) Not an issue, common practise.

2) If you import model in to a game engine it will be always triangulated. Ngons exist only in 3D authoring apps.
So for non-subdivison game assets technically they aren't such an issue, at least not on a flat surfaces.
But.. what if your customer wants to change something.. he will have to deal with ngons instead of you.
If you have ngons in your geometry make a note about it in the description and/or
include wireframe image of that part so customers know exactly what they are buying.

3) In native file format don't apply modifiers.
In other file formats you can export it both with and without modifiers.

7) If you use pbr materials and textures and you know what you are doing it should look same in any software.
If you are using values instead of textures you can include text file/pdf with these material settings.
In game engines textures are always down-scaled and compressed so it will usually not look as crispy as in Blender but close enough.

8) If you are copying someone else design it's always copyright issue.

4. Generic office model is a very bad idea.

5. Unknown how this will affect on demand but with concept models you will definitely have much less competition.

6. Adding a rig can add value only if it is a high-quality actual model with correctly configured animation.

1. Some customers want visualy accurate outer surface. They don't care what is happening in invisible part of model. Some demanding customers might curse you because of excess of polygons.
2. Knowing geometry and topology is very important in 3D modeling. Try to learn how to fix n-gon issues.
3. Sometimes i upload both subdivided and subdivision ready models. This way a customer can choose whether he'll use "finished" subdivided model or he has something else in mind.
The problem may happen when you upload not-subdivided model and it is not suitable for subdividing. Therefore... Make sure that geometry and topology are suitable for the model you're providing. Make sure to emphasize in your description that model is not subdivision ready if it doesn't qualify for that purpose.
4. Scenes are the part of this market. Just make sure that all the models in the scene are made on your own.
5. Explore the market...
6. I have (maybe?) one rigged model. I'm not into rigging nor I'm willing to learn it that much to call myself a "rigger". See for yourself if you want to enter that area and be a master of it.
7. Agree with IndieArt.
8. Some companies agree with that, some companies don't. Internet is your friend.
9. Just use the blueprints from the internet. Anyone who made precize model of... anything, used either blueprints or had actual product to take measurements. I guess most modelers of plains didn't have the "actual product". If they could lay hands on blueprints, there is probably a way that you can also lay hands on blueprints.

If you're a newbie, give yourself a time. Learn. 3D modeling is not an easy task and is not quick money if you're doing it properly. It can also be very rewarding if you study it.
If you sell a "lousy" model for 2$/month, you'll get 24$/year and probably some bad reviews. If you *learn* one year how to properly make a model, you might probably sell it for 10$ or more. So... Without learning, you'll get 24$/year for one model. With learning, you can get 120$/year. One year of being patient student can give you multiple earnings in next year that will also cover the "non-selling" year. Think about it.
Also, find yourself a niche that is comfortable for you. Do you find yourself in low-poly? 3D printing? Rigging? Rendering? Animation? Simulation?