It's possible you might have some weird type of agnosia affecting your ability to capture three-dimensional space in drawings.
I wouldn't say you need to master every area of fundamentals. I mean, specialization is a thing? Colorists don't need to know how to capture the form with lines (though I'd guess most good ones do). Pencillers don't need to know how to color amazingly well.
It's like music. If your passion is all about singing, the only skills you absolutely need are vocalist skills; it's not required for you to know how to compose your own songs, or how to play the accompanying music. Though if you can do all of that, of course more power to you.
You do need some perspective skills so that you can draw figures in perspective, but eh. If acquiring that skill is not a priority to you as a 3D modeler, then that's the way it is. We all have limited time, and we all have our priorities.
Again, tranparency is key IMO. If you were to mislead people about how you do it, that would be bad -- but NOT because the process itself is bad. Hell, you could trace EVERYTHING including the characters, and it would not be unethical in my eyes, as long as you're being transparent and are not ripping off someone else's work.