A hint you may be able to use...
For some time I've used pop bottle plastic as fender extentions. It seems to me that if you cut and fitted a little length that fits between the two rear parts INSIDE the caliper arch and on the inner side of the fenders it would help to divert the watter and mud hitting the slip joints in the caliper. If the plastic is only attached to the back part or even if it's attached along the center line to both parts it'll easily flex with the caliper movement and with some careful sizing and trimming it won't flex into the tire.
If you find that it works out as I suspect it would then do the same at the front and add on the front extension. I know even from running a fender that didn't have much forward extension on the front that a helluva rooster tail flies up and then back into my face if I go too fast. I've since put on a proper fender with more front reach to avoid this.
Anyway, good on you for the ingenuity shown in all the adapting.