Theories on stars and imprisonment.
1. Why does a thousand years have to be a long time? If you are immortal compared to your lifespan it would not even be a blink of the eye. Maybe sending Luna to the moon was like putting a child in the corner or sending them to their room.
2. While she was able to send her sister to the moon, because Celestia is only one pony even if god tiered she was not able to fully draw out the power of the EoF. To fully use them and thus reform Luna you need a pony for each element.
3. Maybe not only do you need the stars to align to release her but you needed the stars to be aligned to reform her. Possibly Celestia could have released Luna at any time but until the stars where right she could not be reformed so Celestia basically put her sister somewhere safe until she could be healed.
Even if immortal, thousand years is a long period. After all, they experience time like everyone else. I doubt it they could just "zone out" for thousand years.
If you'd look back on yourself (being immortal), doing nothing for thousand years, yeah, I guess you would find it a breeze. (Nevermind the mental and memory problems that could occur). However, actually doing shit-all for thousand years? That's still a long period, immortal or not.
EoF? Elements of Fun? Frivolty? Festivity? Fecality? (PROTIP: It's Elements of Harmony).
Also, going on with the words like "reform" and "heal". This is implying there was something to heal in the first place. Yet there wasn't. Nowhere in the show is it referenced or implied that Luna wasn't herself during her Nightmare Moon time. She was simply in rage mode, so to speak.
[/functionzero ranting again]