Here is what the ulema say about praying sitting, lyeing etc:
In the obligatory salat, one has to pray standing if he is able. If he is unable, then he is allowed to
pray sitting. He can pray sitting only if he is not able to pray standing. If he is unable to pray sitting, he is allowed to pray lyeing down. If not lyeing down, then by eye movements.
In the salatal tatauwau (nafl and sunnah) one can pray sitting or lyeing down, even if one is able to pray standing.
However one gets 1/2 the reward for praying sitting, and 1/4 for praying lyeing down.
In the case of a person being unable to pray standing, the ulema say that he will get the full reward, and it will not be diminished, because Allah does not hold people to account for something they cannot bear.
This issue is found in many fatawa by our noble ulema, and as far as I know, there is no difference with the ulema
And Allah knows.