Nikodemus wrote: If an orthodox priest baptised me with sprinkling I would have to receive this mystery again in a valid form. This should also be the case if I knew the priest or bishop to be a heretic.
You are thinking as catholics. The question is not about validity which is catholic words but about Grace. Does such baptism provide Grace?
In many Orthodox countries, in the past decades people were baptized with 3 pouring of water... Are you going to tell they were not Orthodox. If you look at imagery in the Orthodox occident, you can see very small baptismal fonts because people used to stand in and many water was poured...
Moreother, you thinking entirely forgets that we are given chrismation and communion later, and these mysteries can give the Grace that such baptism would not have according to your opinion... That is why it is entirely possible to receive a Christian heterodox by chrismation only as stated in the Canons. One Orthodox mystery is enough to give life to the dead previous mysteries.
Really, you are entirely in Donatism...