(1) How should I perform sajdah? By putting my hands (palm) on knees or the hands should be kept somewhat forward to my knees without support of anything to my palm?
(2) In jamat where should I keep the chair? The back legs of chair in the line of the other people in jamat or the front legs of the chair in the line. In second case the person in the second row of jamat behind me feels difficulty in sajda.
بسم الله الرحمن الرحيم
If you are unable to do ruku and sajdah then it is valid for you to offer salah on chair with gesture. You will keep your hands in the same position as kept in qa’dah while making gesture for ruku and sajdah. i.e. you will keep right hand on right thigh and left hand on left thigh in a way that the fingers reach the knees and keep the fingers straight towards Qiblah direction and don’t hold knees. However, here it is worth noting if you can offer salah sitting on ground in any way (for example in cross-legged posture stretching your leg) then you should offer salah while sitting on ground, it is better.
ويضع عيناه على فخذه اليمنى ويسرا على اليسرى يبسط أصابعه مرجة قليلاً جاعلًا أطفرافها عند ركبتيه ولا بأخذ الركبة الخ (در مختار مع الشامى 2/216، ط: زكريا)
(2) Keep the hind legs of the chair on the edge of the row in which you are standing.
Allah knows Best!
Darul Uloom Deoband