According to my studies, threatening to banish someone to the moon, just for disagreeing with them, is not part of the friendship handbook

which one?

the original one from the show?

or the HD creepypasta one?

yep, immortality is both a curse.....and a blessing

but it's mainly a curse. imagine having friends, or a loved one, and seeing them grow old, and day. while you remain alive, and young for centuries to come

well, 1,000 years ago celestia didn't know what to do with luna, and didn't know how to help her. so she sent her to the moon

same with discord. but.....celestia, and luna most likely came from a neighbouring country, who feared that discord might spread to different countries, so they were tasked to stop him

both of these scenarios, they didn't know any other way to stop/help nightmare moon/luna, or discord

since twilight is the princess of magic/friendship, it's easier for her to help redeem villains, as long she she thinks they can be redeemed

with starlight, she never wanted to destroy equestria, she just wanted to stop twilight and her friends from ever getting their cutie marks

and once twilight has shown what happens if none of them gets their cutie marks. she does feel remorse, and guilty about it

if she didn't feel remorse, or guilt about it, then i do believe twilight would've stopped her another way

well...again, it's an alternate universe

just because they are one age in one universe, don't mean they're the same ages across all the universes

if they were the same, than all universes would have ponies