It sounds like you want to swap the 2015 battery and trackpad into the 2014 palmrest.
Firstly, the batteries are glued in, so you will have to remove 2 batteries to get to the trackpads, without puncturing them.
Secondly, the mounting points are different between the years, so there will be no way to secure the 2015 trackpad if it does fit (the 2015 trackpad uses a number of T4 screws around it to secure the backplate to the palmrest, the 2014 uses 12 screws at the top of the trackpad, since it moves on a hinge essentially).
Also, there is no way to do a straight swap of the palmrests, since the trackpads are different, but it sounds like you know that already.
Cancel the order for the 2014 palmrest, you won't be able to make it work with the 2015 unfortunately, other than doing a botch job with the trackpad :(
The keyboards aren't that difficult to replace, as long as you remove all the outer rivets it will be very stable (even if you mess up a few in the middle). Just patience and some wire cutters are required. Either do this or find a 2015 palmrest (you can buy these with the trackpad and battery or with just the keyboard). The trackpad doesn't need calibrating if replaced, and if done properly you won't have battery issues.