1) Can a believer pray sitting on the floor instead of standing? Would the salah be void if done sitting on floor? I am aware and been told if one can stand but can't go down to sujud on the floor then it's better to pray sitting on chair so you can stand up for the standing position of each rakah while doing the rest in chair?
However, what if an individual instead has the capability to do as I mentioned above but just like in the above scenario the difficulty is getting up from julus, the sitting position on floor, to standing. but instead of praying in chair so that they can stand up this individual does the whole salah while sitting on floor in the julus position. for example sitting straight up for the standing position, bending over a little for ruku and then proceeding to do sujud.

2) If you know someone is doing one of their wajibat wrong but you know they wont be able to change their habit either out of ignorance or illiteracy, should you still inform that individual he is performing his act wrongly?

1) We do not have to pray on chairs. If you are unable to stand up while praying, or it causes you pain or extreme difficulty, then you can sit on the floor and pray. If you can stand for a little bit, especially for the Takbirah and before going to Ruku’, then you must.

2) In that case you are not required to tell them.