I am not completely clear about the issue. But if my thinking is correct..............the stubs are too short.....I saw what I think is an inexpensive sort of internal flare tool you put on a drill to open up the tubing so the OD of that size fits into it. Looked like it could work. Check YouTube but I forget what it was called swedging tool? The copper has to be soft but if you can heat the stubs without burning the hosue down you can soften hard Cu tubing. but maybe there are elbows that might desolder? Then clean it up and solder and you got longer tubing, but I suppose it might then be too long
There is a method of joining copper tubing where you use a plug to expand soft copper to a size that will accept another section of pipe inside it, like a male and female arrangement. It's called swaging or flaring and is also used in older fuel lines in vehicles. There's a brand of high quality pipe fittings called Swagelok, but here I think swaging refers to fitting two part ferrules to a compression fitting using a gauge to tighten down.
Basically, it saves on fittings and I haven't seen them in recent years as there are much better methods of joining pipes.
Looking at the images, these shower bar fittings are separate rather than being attached to a single bracket. They work by having the supply pipe usually 15-mm sticking out of the wall as an elbow. The brackets shown fit over the 15-mm pipe and an adaptor that ends up as 3/4" BSP acts as a compression fitting so all the work can be done from the shower side.
What you can see in the first image is the 15-mm copper pipe and the olive of the compression fitting.
If the olive won't come off, and it shouldn't, you are stuffed for adjustment. However, with care, you can get the ferrule off by cutting through it. If there's room, you can use a tool called an olive puller.
There's a lesson here. Get your plumber and tiler involved before you start any of the work. Finding problems at this stage is not a good idea,