1. You should be able to choose what potion effect it give you like when you first craft the instrument and play it no effect is given so you just can have a normal instrument if you want to. Also if you want it to give you a potion effect you have to put the instrument and the potion you want it to give you the effect of in a crafting table together then when you play ityou are given the effect.
2. I think you should be able to play it for has long has you want to like when you hold right click it just plays over and over agian and when you let go the potion effect is given.
P.S. if you don't like the first idea can the ukulele give you resistance plz.
Mod creator if you add Clarinets to this mod try to make it have 4 stages: 1. Squidward's Clarinet which gives you Nausea (because it is the clarient not squidward). 2. Clarinet (regular) you can decide on the buff for it. 3. Nether Clarinet. Again you can decide on this one and finally Ender Clarinet. Which you can decide on the amazing buff.