............ seventy or so years prior to the series current, there was a war.
after nightmare moon was banished, celestia bulked up her armed forces as likely aggressive factions saw that they were down one demigod.

this is not a first war.

also wouldn't they be using an element bearer as a bargaining chip? she would be far too valuable to keep locked up or detained.

Thank you for fast update! I wuv you <3

4225130
1) I do not remember that happening whatsoever
2) They could, but I didn't want that to happen soooo...no. That's not what she's doing. Instead she made it seem like Rainbow died.

4225147 was brought up in the comics with the war, a time refrence using a poyn that helped her during that war being around as an old mare around Granny Smith's age.

in testing testing 123 they liuterally stated that celestia built up the army after luna was banished. while they did not say war or enemy threat, what other reason would she do that?

and speaking of which............... luna. luna would know whether or not rainbow dash is alive.
also saying you do not want it to is not a good reason for it to not happen. you are putting the narrative over the charactes, and that does not often lead to good results.

4225226
This came out plenty before that episode and the comics and I'm not going to change it now, also i is perfectly fine to choose how I want to write my story as long as it is believable. You can not say for certain that Luna would be able to know, there's this fancy thing called MAGIC that can do a number of things to stop that.