The author's attempt to answer some of your questions.
Yes; that's why I included a light button, which allows you to have constant illumination, with a slider to adjust the illumination angle. It's on the top of the screen, in a prominent position. It's also mentioned in the help for the app.
No; having one or two apps per planet is an approach that is not 'scaling' well. But I may try to tackle the problem in a different way.
Yes, that's true. Keep in mind that most real-time Moon simulations don't have topographical lighting at all. There is a tradeoff here: you can have the Moon rendered with perfectly accurate lighting, or you can have it rendered fast so that you can interact with it. You can't really have both at the same time today, not on a mobile device - maybe not anywhere. I spent a lot of time looking through a telescope comparing Moon Globe to the real thing, so yes, I know it's not perfect; it's just the best approximation I could manage.
This is explained in the product descriptions on the App Store. Basically, the HD versions are for people who've already tried the free versions and would like slightly (not extremely) better resolution and rendering. If that's not you, please don't download them.
Because compiling and writing this material takes a lot of time, and I just happened to be more interested in Mars.
Email midnightmartian @ gmail.com. Please keep in mind that the guy who does Midnight Martian loves to hear from you (usually) but is very busy making more apps and features.