1- It means you can pray it any time during the day or at night, and you can count it as the Nafila. So let’s you want to pray the Nafilah of night, Salatel Layl. It is 11 units, so you can pray Salat Jafar al-Tayyar, which is 4 units, and count that you have prayer 4 units of Salatel Layl, then you do 7 more and you would be done.
2- Yes you would be considered as completing the 51.
3- You can do that and it would qualify as the nafilah, but you may not get the full effect or praying Salat Jafar al-Tayyar. In any case, you don’t need to be on wudhu if you want to recite the 300 tasbih after the prayer