My question is this: how can Rami bar yechezckel bring a proof to the din of tzavoas shchiv m'era from Achitofel, if achitofel committed suicideand was never a shchiv m'era? also even if yes could it be that chazal would include in their takono someone who commits suicide?
ProcureStaff, A Volt Information Sciences, Inc. Company
560 Lexington Avenue, 10th Floor
New York, NY 10022
The ROSH (9:18) proves from this Gemara that even a Yotzei b'Kolar, a person who expects to be put to death soon but is presently perfectly healthy, is treated as a Shechiv Mera, and if he distributes all of his worldly possessions the recipient acquires without making a Kinyan (and the giver may retract if he survives). As you write, the Rosh indeed seems to equate a suicide who is about to kill himself to a Yotzei b'Kolar.
The Rosh cites the RIF who says that a Yotzei b'Kolar is Metzaveh Machmas Misah (and even his partial gift can be retracted), see Pilpula Charifta there #3.