JNA was always confined to barracks. There were very few attempts to break. JNA never anticipated local population becoming unfriendly so it was utterly unprepared. JNA high command opted for de-blockade by sending relief force but this was often botched and badly executed. Also, troops were green (19 YO kids) which were demoralized and were in poor fighting shape. Only after mobilisation did some units gain fighting spirit.This doesn't apply to Airforce because pilots were officers not green kids.Was Ljubljana captured by JNA or was it expelled from there?
Do somebody know how the ethnic groups were divided in the brigades/units? Was there mostly Serbs, had the Slovenes and Croats left the army before the war started?
Officers were in majority Serbian (hard to give percentage). Historically Serbs tended to see army service as something honourable. Example: in the Army school initial intake was matching Yugoslav census but by the end of schooling many non-Serbs dropped out. Also other ethnic groups started leaving once political tensions rose. When mobilisation started JNA sent recruitment notices too all nationalities but mostly Serbs responded to it which only cemented Serbian majority. This led to a rise of Serbian Army. I don't remember any place where a complete unit (in game units) switched the sides.
I would have events to demoralise JNA units (e.g. Slovenian government recalls it's officers from JNA) to keep game going otherwise it's all done in a week.