Here's a suggestion: purchase a cheap but very functional SLX 12-speed shifter. I haven't heard of this model failing. I think their internals are different. They don't have all the extra features of XT and XTR but they work really well and completely interface with the remainder of your drivetrain. When you finally get the warranty replacement for your broken shift lever, keep the SLX as a spare just in case you have another failure. I did this with an XT shifter backing up my two bikes with XTR. But SLX is likely to be more reliable long term than XT, especially if you only need to put it on your machine once and a while. Yes, this whole thing is a frustrating load of BS, but I've stuck with the new Shimano system because when it's working it's, in my opinion, the best functioning mechanical MTB group out there.