If a person has many years of qadha namaaz Is it okay for the person to recite nawfila (mustahab) salaat as well? Or should they complete all the qadha prayers first.

He can recite nawafel.
There is no problem.

If a person has many years of qadha namaaz. Is it okay for the person to recite nawfila (mustahab) salaat as well? Or should they complete all the qadha prayers first.

There is no problem to recite the nawafil and the qaza alongside.

If a person has many years of qadha namaaz. Is it okay for the person to recite nawfila (mustahab) salaat as well? Or should they complete all the qadha prayers first.

He is allowed to pray nawafil but better to pay qadha which is wajib on that person.