بسم الله الرحمن الرحيم
(Fatwa: 1302/1114/N=01/1440)
If you cannot offer sunnah muwakkidah waqtiya salah with qaza and ada fardh salah due to weakness then you should offer sunnah muwakkidah while sitting, and it will be valid without any doubt. It will not be only nafl salah rather due to excuse and compulsion you will get the rewards which one gets while offering in standing position, in-shaAllah. However, you should offer two rakah of Fajr sunnah muwakkidah standing because as per the correct and preferable saying qiyam is fardh in the sunnah muwakkidah of Fajr salah like fardh and wajib salah.
It should be noted that you need only to offer qaza of fardh and wajib (witr) salah not the missed sunnah salah as there is no qaza of missed sunnah.
ومنها – أي: من فرائض الصلاة – القيام ……. في فرض وملحق به كنذر وسنة فجر فى الأصح لقادر عليه وعلى السجود (الدر المختار مع رد المحتار، كتاب الصلاة، باب صفة الصلاة، 2: 131، 132، ط: مكتبة زكريا ديوبند)، وأبدل النذر فى الخزائن بالواجب (رد المحتار).
Allah knows Best!
Darul Ifta,
Darul Uloom Deoband