So what do you call the name Larix xmarschlinsii?
The name of a notho- (i.e. hybrid) taxon, more specificially of a hybrid
arising spontaneously in cultivation.

So it is the same as xFatshedera lizei.
Now where do we put xCitrofortunella mitis (Calamondin)? Is there some location
where the natural habitats of tangerines & kumquats overlap, or did it arise in
cultivation in the distant past?
One of the interesting aspects of hybridizing, at least in orchids, is that
occasionally there is a registered artificial hybrid which is subsequently
discovered to be also a natural hybrid. The latter is published by a botanical
taxonomist, following which the poor plant is doomed forever to carry two
different names. According to RHS rules. If it is used as a parent in
hybridizing, it is called by the registered name. If the natural hybrid
discovery comes first, the plant only has one name, but it may be written
differently or carry a Latin ending.
Iris,
Central NY, Zone 5a, Sunset Zone 40
"The trouble with people is not that they don't know but that they know so much
that ain't so."
Josh Billings (Henry Wheeler Shaw), 1818-1885