Generally what I was thinking too. OTL, Yugoslav AFVs were predominately French, starting with FT-17s, and later on they also acquired some R-35 tanks, as well as Tank Destroyers of all things, 8 Czech Š-I-D tankettes carrying 37mm AT gun. FTs were a mix of both originals and model 1928, though how many of them were in actual working condition is hard to say, and there were 54 of them in total. R-35s numbered 56 vehicles, and they were the most modern tank availlable to Yugoslavia at the time.
Now what vehicles Yugoslavs might get their hands on from the French is an interesting question. I really do not see them going for B-1s, putting aside its combat performance, it is a large, complicated and expensive vehicle, not really something Yugoslavia would choose IMHO. There is also a question of mobility, since Yugoslav Infrastructure was not as well developed, and considering rather harsh terrain in some parts of the country, these B-1s could easily end up immobilised without a railway nearby. More likely option might be various French Light Tanks, such as R-40 and H-39, simply because they would be cheaper to operate then alternatives, and French could likely sell them at rather low prices, if only to restore some of their lost influence in Yugoslavia. Another AFV that might catch their eye is the Armored Car, and French Panhard 178 or its successor the Panhard EBR, as a decent option to mechanize their cavalry formations.
Other vehicle that may also end up in Yugoslav service is the humble Renault UE, to serve as artillery tractor, and to mechanize AT and light artillery batteries. Here France is not the only option, as Romania had a license to produce them as well, and without France falling the production would continue for a longer time then IOTL, and some may yet end up in Yugoslav hands.
Of course, they still need something heavier, more heavily armed and armored, and couple of years after the end of the war, we could see Somua S-40/Renault G-1 (or was it G-2?) series of tanks entering into service, as French army is demobilised fully and reequiped with more modern arms.
I would argue that it was not so much of a mistake, since Banovina was merely a more autonomous province of Yugoslavia. It did have much larger freedoms then some other nationalities and regions, but it was still firmly suborned to the Yugoslavia. Though, ITTL we may see some further developments occur, as Maček and some of his compatriots hoped that Croatian Homeguard of WW1 vintage may end up revived, and serve as a distinct part of the Royal Yugoslav Army.
The basis of this new Homeguard was to be armed detachments of HSZ,
Hrvatske Seljačke Zaštite (Croatian Peasant Protection), an paramilitary wing of the HSS,
Hrvatska Seljačka Stranka (Croatian Peasants Party), which numbered roughly 200 thousand members in 1941. True fighting strength though, was the HGZ,
Hrvatska Građanska Zaštita (Croatian Civil Protection), a mixed regiment sized force composed out of 3 infantry batallions, numbering roughly 1000 men each, as well as a cavalry squadron and a company of motorized infantry. They were exclusively armed with light weapons, and motorized company used motorcycles with sidecars.
Once political tensions calm down, and war ends, we could see not only Croatians getting their own units, but also Slovenes, perhaps in their own Slovene Homeguard. The height of irony is that Draža Mihailović, the man who led Četniks OTL, argued in late 1930ies that Croats and Slovenes (and Serbs OFC) should serve in their own ethnic units, and we could really see something like that happening in the postwar period, as Yugoslavia liberalizes somewhat.
