The movie gives us colors for the two silhouettes and the one that you're assuming is Kirishima has yellow eyes with a purple aura around his body. Likewise the Spikey haired one has red eyes with a red aura around his body.

crimson riot stated that his reason for his attitude was because he hesitated and couldnt save someone he said this was during his internship as a sidekick

Small nitpick but Crimson Riot said it was just as a sidekick not an internship.

if kirishima is crimson riot than he will soon fail to save someone which might mean he tries to use eri to send him back but it goes to far.(or already did in a parallel timeline if so it was probably fatgum who died without because crimson riot did not exist to inspire him)

This does not sound like something Kirishima would do at all. Like this sounds unbelievably out of character for him, to force Eri to send him back just to save one person. It would be more in character for him to either get depressed and have his friends help him through it or just keep pushing foward.

crimson riot has red eyes like kirishima.

I might be wrong but doesnt Crimson have all blacked out eyes. I don't remember too clearly.

eri's horn has grown back recently and if it keeps growing she's going to need to release a lot of power very soon.

Eri's power is to revert a person's body to a previous state. Not to time travel. It would take a massive asspull/powerup for her to go from only affecting a person internally to a global setting.

they are both the red light 3rd bearer and one for alls brother isnt the original bearer. he is the blue light (second bearer) and the white (possibly representing purity of the power) is all for one, who doesnt appear as a vestige because

All For One's brother is the original bearer though, the quirk didn't get created until the stockpiling quirk fused with his quirk.

if one for all can be given to two people at once than its possible that it can also branch off and form new lines, if they both recieve the quirk at the same time. what if they pass it down to different people? what if their exists a second line of one for all users in the present? (not necessarily in japan)

No. Theres only one OFA two people cant both have the quirks.

Also the only silhouette you can make a case for is the Spikey haired one vaguely resembling Bakugo. The other one looks nothing like Kirishima.