I don't think, that reducing moving range oh amphib Transports would be a good solution because it wouldn't only hinder Micro landings (which are the real problem) but also big invasions with three or four corpses, an HQ and so on. These big operations should always be possible.
Why do you think, small units in every coastal town (I don't think you need one every coastal Hex to substantially restrict micro landing by detachements) would be ahistoric. I think small garissons, alarm Units build of militia, police etc. could have been in every town and resist a (weak) Landing operations (by Detachement) until better units arrive.
I even don't think, that garrisons couldn't solve the Problem. You don't Need to occupy every coastal hex, why do you think so. Of corse can a Detachement even land on a no-town-hex but it will have supply 0 in the next turn and can be blocked even by some garrisons. Maybe it is not a complete solution für Micro landings, but it would make Things at least much better.
Naval Mines: Not every mile coast in Germany whas guarded by mines, that's right, but the entente did not know the places where they where layed and the danger by mines was so big, that English ships didn't dare, to drive near to the german coast. And in the baltic sea Germany had to make a huge afford regarding russian mines in the 1917-operations. So such a solution wouldn't be extremely ahistoric, at least not for German coasts, russian coasts in the baltic sea, Italian and Austrian coasts in the Adriatic and Turkish coasts near the dardanelles.
Another solution would be to make detachements no longer transportable by amphibic transports but only normal transports (like garrisons in the ww2-games). So an invader would always have to use a full corps, which is not everywhere at every time available and much more expensive...