"My grandfather loves to watch TV from his armchair."
Translation:Min farfar elsker at se tv fra sin lænestol.
I think it should be allowed, the English sentence doesn't say exactly whose armchair it is, so it could be the subject's or another man's.
You should use 'sin' instead of 'hans'. If you use hans it is someone else's armchair
Nothing, in fact it's what is used as the main Danish version of this sentence.
Nothing wrong with "bedstefar" - which is even a better translation since there is no clue in the English "grandfather" as to which side of the family he is from. In Danish you often use "farfar" (father's father) or "morfar" (mother's father), but "bedstefar" covers them both :-)