So, Deadly had a good point about lines I think, if you're going for show accuracy. Also, you're right that references are important; I think for legs and wings, they're doubly so, at least until you've drawn quite a few of them!
That said, I do really like this picture; it's a fun crossover and Derpy as an Inquisitor makes me smile.
Re: guide lines - Yeah, I learned that lesson after drawing Twilight and Pinkie...I did use them on RD's head though, and it definitely made me feel more confident about eye placement and size. I totally encourage everyone to use them in the roughing out stage!
I really like the self portrait as well. There's something in the face and goggles that makes it seem very expressive. Also the beard is quite well-done! I don't know much about shading and depth when it comes to drawing from life, so I don't know that I can help you there, except to encourage you to keep practicing
I will say in regard to time that there is a noticeable difference in quality between something you work at for an hour and something you work at for two hours, but I understand the attention span issue. My advice would be to do things in stages - rough in first, then walk away, then come back and add details, then chat for a while, then go back and refine, etc... You don't have to finish in a single drawing session, or even a single day.
Also, everyone please feel free to post works in progress here as well! That way, we might be able to offer advice as things come together, and you won't have to feel like you MUST FINISH TONIGHT OR ELSE!
The only person that can keep you from being good at drawing is you! We're all just here to encourage you and provide another set of eyes and experiences to help guide you toward improvement! So don't believe the Lie - you can draw, if only you'll put your mind to it!