Are we allowed to perform Mustaab / daily nafal prayers( Namaz e Shab, Daily prayers nafals etc) sitting down with or without any valid excuse. Would we get half the reward? As I heard that nafal or Mustahab prayers can be performed even when you are walking. Could you kindly advise me according to Shareya and my Maraja's Touzeeh. I follow Ayatullah Khamanie.

Yes we can and it’s fine even without an excuse. This is not the case for wajib salah though. Allah knows best about the reward. Ofcourse it’s best to do the prayer standing.