1) When Praying on the chair during tashahud is it must to have your feet on the ground flat 100% or if it’s a little up it’s alright ?

2) When doinh ghusl can I wash the face and head followed by full body rather then right followed by left

As long as you are described as sitting it’s fine.
According to Ayatollah Sistani, you can do the body after the head