1. Boolean - Unit has specific buff and Real - Game - time of day.
2. Make the buff be granted by an aura ability. You can put the aura into a hidden spell book to hide the comman card icon. Don't ask how to use a hidden spell book, use search.
3. Transform ability (Bear Form for normal units, Metamorphosis for heroes). You might need to use Engineering Upgrade.
4. Importing should be straightforward, open import manager (F12) and import the file. Change the file path to a texture you want to replace.
5. You are propaly asking for Environment - Set Sky.
6. Yes, with JNGP editor. Upto 480x480. However I do not recommend it since it takes ages to fill with quality content. Don't be too ambitious especially when you are a beginner map maker :)