the german crosses are mainly maltese and the Russian one of the imperial crown is a maltese one
It's not the Imperial Crown that is the issue, but princely caps.
this cross looks latin to me. Straight cross bars.
A Greek cross also has straight arms (though of equal length).
I am almost one hundred percent certain that you are wrong that it was a Muslim prince and you can now take a look at two copies in Russian prince coat of arm. I am sure-fire that you know the Princes Yusupov family
I am also quite sure it's a normal cross on a normal prince's crown, but just to clear things up:
The Yussupovs were once Muslims (hence the crescent in their arms), but had converted to Orthodox Christianity a long time ago. Thus it's natural that their arms feature a cap with a cross. The princely cap without cross I mentioned belonged to some Russian subjected princes (on the Crimea or Central Asia) who still were Muslims, as far as I recall.
he quarterings are a bit troublesome to me, in that they ALMOST seem to be mirror images of each other, looking at the rampart lion/leopard repeats. I may be incorrect, but I am seeing this shield as bearing the quaterings/charges of TWO people. Thus for a union of two people to have such similiar quarterings is interesting, to say the least, IMO.
Agreed. You would expect the quarterings to be:
1, 2
2, 1
and not
1, 2
1, 2 like here.
And the mirroring issue is very odd too.
I would say that points to Eastern Europe, i.e. a complicated coat of arms that was designed from scratch.