In the disappearance of Haruhi Suzumiya, who pushed Kyon down the stairs?

He wasn't actually pushed down the stairs. After the future Kyon got the needle gun and got everything back to normal, Yuki altered everyone's memories so they thought that Kyon was pushed down the stairs, while he himself had no idea about it. Yuki just moved him to the hospital because he was still injured and basically healed him. So no, none of that was a dream.

Havent seen it yet sadly

(I might be wrong) They hinted that it was Yuki. She went back to make his sudden hospitalization seem to make sense.

hev was not pushed by any one

If it did happen, It would have to be Emiri Kimidori. She's also an alien, and while she does not interfere, she is also part of the observation team. She also has the power to disrupt the memory of others. it was confirmed that after the events of the movie, Yuki going rogue, Emiri was assigned as her chapperone. So it's most possible that it was her that either pushed him, или the one that created such a memory in the other club members.