Galliano's accordion is really old (my guess is between 55 and 60 years). In that period (early to mid 1960's) it was quite common for Italian accordions to be tuned 443Hz. Later they settled on either the international (ISO) standard of 440Hz or on 442Hz, used quite a lot in professional orchestras (and chosen because string instruments play louder at 442Hz than at 440Hz).
When I started with accordion many people locally had Crucianelli accordions, likely also 443Hz. Later when I started doing accordion tuning I changed it to 442Hz (and also reduced the tremolo).
So I'm not surprised that Galliano uses 443Hz.
Changing the overall tuning of an accordion isn't good for the reeds, as the scratching makes them weaker. If you want reeds to stay good for 6 decades yet have your instrument tuned regularly (because it does go out of tune) then it's best to not do a major retune. And afaik he only gets his accordion tuned at the Victoria factory and they are adamantly against changing the tuning of a whole instrument I guess he will just stick with the tuning he has. The factory has very limited stock of the old original reeds, so he also needs to avoid breaking reeds often.
Concert pianos are often tuned 442Hz, and that's about close enough to not be a big problem.
For instance, in "Mare Nostrum" (a concert I attended in Antwerp) Galliano plays together with trumpet player Paolo Fresu and pianist Jan Lundgren. If you listen carefully you can detect tuning issues between the instruments but it's not disturbing. A 1 Hz difference is about 4 cents and that's not more than the tremolo you already get when playing chords on a single instrument of from playing octaves on an accordion that needs tuning (like hist Victoria in the YouTube video...
In that period (early to mid 1960's) it was quite common for Italian accordions to be tuned 443Hz. [...]
When I started with accordion many people locally had Crucianelli accordions, likely also 443Hz.
The accordion may have originally been tuned 444Hz (that was common for the American market) and then gone up a bit over the years. It is unlikely that an accordion would go up by 2Hz over a few decades, but 1Hz is possible. (Reeds more often go sharp over time than go flat.)
RE the piano question . . . somehow one might just throw out a wild guess it's a fair bet that "the most in-demand squeezeboxer in the western world, if not the planet" has, oh, one or two other accordions lying around, and is not limited to one hz443 box . . .