NSEU
Even if slush is only transferred into each main tank at rate of 200 kg per hour, then in 10 minutes that is 33 kg in each main tank.
If all, or even only part, of it freezes on contact with fuel that is a lot of potentially clogging debris in each main tank relative to the number and size of inlets. In context, the AAIB have highlighted for consideration four or so foreign objects in whole fuel system which are minute in comparison. I wholly agree that if it does NOT freeze in each main tank and remains as water then it becomes an irrelevance.
I have no idea which main tank inlet would be affected first in this most unlikely scenario which as you say requires the blockage of all inlets at least partially. I expect that eddies particularly from the operation of the the water scavenge pumps might push debris around unless/until it became trapped. Also I expect that the suction feed inlet might be blocked earlier without any noticeable effect - particularly on the right hand side given the foreign object.