Everything will bolt up. Did you take a good luck at the back of the transmission and make sure the case isn't cracked the rearmost edge? I had one that pretzelled the dogbone mount and the transmission was pushed back into the rack and pinion cracking the case.
Both engine and transmission will bolt up and work with the 2015.
If you really want to wring every last mile out of your current drivetrain I'd pull the oil pan off the transmission and have a look. If there's nothing to scary looking stuck to the magnets then I'd consider
swapping out the valve body with the one from your donor car. It's a pretty easy job and typically the oil pan gasket is re-usable.
The transmission in my 2014 was surging fairly often and was giving the previous owner some other problems before I bought it. I planned to replace it one day and finally did when the engine cratered at 280k km a few years ago. After tearing the old transmission apart I'm pretty confident that I could have just replaced the valve body and kept that old cvt going a while. I did consider trying that route but didn't want the iffy transmission to contaminate a good valve body if it did decide to self-destruct.
As for whether to replace your engine or not, that's your call. There's a 4 Mirages in the fleet out here with over 300k that are still running the original engines. 2 of them still have the original cvt.
Here's how you remove the valve body. Skip to 4:00