>>>I think it's the shape of the hull that jets water up somehow, whether it hits part of the engine and up I'm not sure
Yes I guess if you imagine an Aerotec towed onto the plane without an outboard there may be a slightly confused meeting of water immediately behind the keel... add in the presence of an outboard leg and unless it all coincides perfectly there's plenty to cause splashing.
Fins probably work somewhat the same as my daughter being in the bow giving a more level or bow down ride. I have a couple of sets in the garage but so far this outboard is undrilled so not keen to experiment.
Really interested to hear the results of your splash shield Max. For me that design using the transom wheel buttons would be another faff at the slip but if the principle is sound you could buy and mount a second set of buttons about at the width of the outer transom pad purely for the purpose of attaching a narrower splash bracket (like the one linked above - post 7 here... Old used Aerotec impressions.
) so it could stay on all the time apart from when you wanted to pack the boat away.