Nashville tuning is when you fit very light strings to the lower three positions on a guitar, and tune them to the same notes E A D, but an octave higher than normal. Another way to think of it is that you are using just the high strings from a 12 string set.
I did keep a guitar in Nashville tuning at one time, but then I realised that what I was really wanting was a 12 string.
I would really advise you to stick with the mando tuned in fifths. You will be amazed how easy it is to get used to the different tuning.
One thing you may not have realised is that a fifth tuning means that the same fingering pattern applies on all strings- so if you play a major scale based on the open string, the fret positions go 0,2,4,5,(change string) 0,2,4,5. Unlike the 4ths and Major third tuning of the guitar top four, that works on any two strings.
Secondly, you can play nearly any mandolin chord with two fingers.
It is always useful to have a less familiar instrument when you are composing- otherwise it is much too easy to settle for the patterns that your fingers are familiar with, and never find anything new or interesting.
There are few things more demotivating than the feeling that "there's nothing I can play that I want to hear!"
Accept the challengep- learn to play the Mandolin properly!
Cheers
Dave