Eh.... 0.7mm at the base of a thread (basically, the start of a crack, with a fitting putting force on it)? Not brilliant in my book.
My call would be something similar to what PeteS said. One of my favourite tricks is to basically create bushings by cutting slices out of the circumference of a pipe and then inserting it inside the pipe's normal diameter.
I mostly use it to go between 22mm and 28mm pipe sizes when a normal reducer won't do for the purpose - one ring like that neatly plugs the difference between the inside diameter of a 28mm pipe and the outside diameter of a 22mm fitting.
For example, this is presently* what the end of the v2 HEAL valve looks like (complete with my crappy handwriting):
It might be a little difficult to see the boundaries between each pipe, but the solder gaps are all just about visible if you expand the image.
*Eventually, it'll have a modified 22mm pipe inserted into the slip coupling, which will contour down from the 22mm coupling, which acts as the sealing face, to the 20.3mm barrel.
If you've got any spare 15mm pipe around, it might be worth seeing if such a "bushing" could be used to solder between 15mm pipe and the head. It'd be easy enough to cut and check the fit.
... actually, it occurs to me that I think I do have some 15mm pipe that I've cut to fit inside its own diameter. I'll see if I can find where it is and measure the ID of that.