1) My father is sunni and I am shia. He knows that I am shia and sometimes invites me to pray salat with him leading the prayer but I don’t know if I should say my own shahada, and should I pray again later or is my prayer valid if he is leading.

2) My other question is, when it comes to ghusl I’ve been doing it my whole life with going in the shower and fully soaking myself and then starting with my head and neck touching them and then turning my right side and then my left, but I didn’t know I should do my head and neck alone first without wetting the rest of my body then doing the right side and left is my prayer not accepted, also is ghusl al janabaa the same procedure?

1) In regards to namaz with your father, you should recite the surahs and every dhikr of namaz yourself including tashahud and you can do this in a low voice.

1) In regards to ghusl, doesn't matter if prior to starting ghusl the body is wet. Method of ghusl is to first wash the head and neck, then right side and then left. Ghusl e janabat is of the same procedure.