As a quick reply to my suggestion earlier about the RSK capturing Split in an offensive, it looks likely that both Split and Zadar could have been captured in a concerted effort (certainly there was a plan to try and get Split, and had that suceeded, Zadar is a logical next step. Combine these with Dubrovnik falling (quite possible) and it seems possible that with most of Dalmatia and a fair bit of the rest of the country lost the RSK could end up at the very least like the modern RS in Bosnia.
If things are as OTL until summer '91 the chance of such a thing happening is beyond miniscule. By November '91 JNA and RSK in Croatia are outnumbered (though they had local numerical superiority around Vukovar and Dubrovnik)and the only thing they have going for them in superior equipment and the momentum that was almost dead. Capturing Split was beyond JNA since summer of 1990, with the armed uprising of Croats in BiH in late summer/early autumn of '91 it was downright improbable. Taking Zadar was still on the cards and offensives were launched by taking of the town remained beyond the capabilities of the unit JNA and RSK had at their disposal and Croatia was sending fresh troops (and increasingly better equiped) to the front each day.
JNA+RSK+Serb militias needed between 60-70K troops to complete the conquest of Vukovar, a town 2/3 of the size of Zadar that was completely surrounded for almost two months and it was only achieved after Croat leadership abandoned the breakthrough towards the town to apease the western diplomats. Zadar had connection with the rest of Croatia, offensive against it happened later in the conflict allowing the defenders to better prepare. So it was speculated JNA+RSK+Serb militias would need around 90-100K of troops to complete the conquest of Zadar, that was just the manpower they did not have.
As I have mentioned earlier JNA draft failed spectaculary(not just in Serbia proper but everwhere) and non Serb soldiers deserted daily, even Serb soldiers diserted in a non-insignificant number. The moral of non-militia units was almost non-existant and there are reports of soldiers and in some ocasions even officers flatly refusing to follow orders to attack/shoot.
The conquest of Dubrovnik in a Vukovar style fashion could have occured since the JNA+RSK+Serb and Montenegrin militias had local numerical superiority and superiority in equipment but in order to completely conquer Dubrovnik the attacker would have to completely demolish the old town. In a case of of a risk of the town falling a plan was made to retreat all the defenders behind the walls of the old town and hold on until help comes or until they are overrun. To break into the old town the attacker would have to demolish large sections of the medieval walls. I just can see the west no reacting to such wanton destruction especialy after the fall of Vukovar and the events that followed them.
The capture of Karlovac is even completely in the sphere of academic discusion.
In late November JNA was a spent force in Croatia. The offensive capabilities have been too dented to provide serious resistance to an organised Croatian counterattack (operation Orkan '91) without support from units in other republics and the Serb population outside Croatia and BiH. The only reason western Slavonia was not completely retaken in winter of 91/92 was the Sarajevo ceasfire. Without the Sarajevo ceasfire and the continuation of hostilities the war in BiH would have looked very differently since the JNA would have a 600km front line to worry about in addition to any opposition in BiH. Dubrovnik might have fallen in such a setting but that is highly unlikely preparations were allready made for a counter offensive that would have relieved the city by mid January '92.
With the units at their disposal in '91 the JNA+RSK+Serb militias were bound to fail, it was just a matter of time when that would happen.
Also the map you posted above is somewhat deceptive because it ignores the terrain.
Cheers