Can I perform the act of Imaam if nobody is willing to lead mass Salah?

I am a man of 61 (according to solar year). I am suffering from Osteo Artharitis ( a disease of bone ) of both knees. I can stand, walk and climb steps, but unable to sit on the floor in any posture that requires folding the knees. I can only sit down on the floor by stretching both legs forward. I am offering my daily slah (namaz) by the help of a chair or stool. In my individual salah use to stand in the quayam posture, bow down from standing in my rukoo posture, then sit down on the chair to perform sajdah by nodding my head forward and qada by sitting on the chair. 1-My question is that while attending slah in jamaah(Mass namaz) if I place the chair or stool in the saf and stand in the qayam posture my position does not remain in the line, rather it occurs one step forward from the saf. Therefore I use to sit on the chair or stool during qayam .Some of my friends opine that qayam is farz in salah, so if I sit on the chair during qayam , my namaz becomes unacceptable(Fasid). Please enlighten me in the light of rules of Shariah. 2-In case nobody is willing to lead mass Salah (Jamah of Farz Namaz) , can I perform the act of Imaam? Because in my house usually my children and I perform individual salah because none of my children dare to perform the act of imam.