You will not be able to do both wudhu' and ghusl simultaneously and at once.
So, you can either do wudhu' before ghusl, or after ghusl.
When you wipe your head during wudhu', you can use four fingers, or three fingers. Both cases are fine.
And Allah knows best.
No, it is not allowed to wipe over the Hijab for Wudhu. Wiping for Wudhu must be on the head or the hair itself and not on cloth covering the head.
Wiping over the feet is an obligatory part of Wudhu, but washing legs is not a part of Wudhu and can not be done as part of Wudhu. You can wash your legs any time but never think that it a part of Wudhu, otherwise it will be a Bid'ah.
Masah or Mas-h مسح (wiping) must be on the central area of the head. If it was done around that area, it will be valid as far as it is not extreme right or extreme left which is away from the middle area of the head.
Not all your head nor all your feet must be dry when wipe on them in Wudhu. Just part of the front part of the head, and one or more figures must be dry. The wiping must affect that front part of the head and one or more fingers. The affect is not transfer the wetness from the hand to the wiped areas.