restrictor is plenty big enough mate tbh its like not having one at that size, im not sure on how much oil should flow cranking but worth checking the oil pressure to be sure.
never rule anything out but i will be suprised if no water has killed it, stranger things have happened at sea though mate .....

at 250ml in 15 seconds that would make you 1 litre per minute that does sound kinda low to me especialy with a large bore fitting like you have.
id defo be checking the oil pressure now if not at the turbo feed at least of your block gallery, if you run of a filter relocator make sure you tap in after the filter not before as it will massivly raise the oil pressure reading otherwise.