No, even in the original install (which I was not a part of) there was no reduction in size.
The new one uses 3/4" CPVC and is 3/4" all the way to the run in the wall (3/4" copper), which I shortened and 90'd out to a 3/4" copper threaded coupling.
None of the couplings reduce size. I actually used the 3/4" copper pipe I removed from the wall for the various bits and pieces I had to add on the supply side, and for the stub out at the top of the 90 out of the wall.
Yes, the straps are (barely) in the right areas, but I had a few issues to deal with there.
Is there any issue with the TPR valve being mostly sideways? I wasn't able to find a reference in the code that was explicit about this, and since they use top-mounted ones all the time, I figured it would work.
As for the BW, there's really only one place on the net that sells them, a quick google search will confirm.
I don't really want to post the name here, as they may not be in the good graces of this forum.