It is a beautiful instrument. It would be nice to have a sound file posted.
Is there a reason why you call it an EPINETTE rather than a SCHEITHOLT ? Are the plans from a French source rather than an American-German one?
If you study ukes in their broader aspects, you will find the "D tuning" isn't anything strange.
Generally speaking, British and Canadian players tune A-D-F#-B, and according to some sources, it was popular in Hawaii at one time. It was pretty much standard before WW-II in the USA. Because of that you can readily purchase string sets intended for the higher tuning. Many small tuners even give you a choice between U-C and U-D setups.
I have several "learn ukulele" books, and probably the best of the bunch is Roy Smeck's classic, which is all in D tuning. See https://www.amazon.com/Mel-Bay-Ukulele-Method-Smeck/dp/0871664836 for details.
I keep one (cheap) tenor tuned each way.
