how the mercenaries come to ... (spoilers)

I don’t know what are the rules for private planes. I am sure the Austrian authorities would have checked the passports of all the Georgian passengers, but would they necessarily have checked the contents of the cargo hold? Possibly not. Or maybe the weapons could have been acquired locally from other criminal gangs the Georgians had links with.

I asked myself the same questions but I didn’t let it bother me. It’s unimportant.

Without detection? No, impossible via air, you can't just casually fly around and cross borders, there is complete radar coverage, you need IFR flight plans, visas etc. etc.
The movie is simply dog shit when it comes to anything but neat combat choreos.

The jet full of armed guys aside, I find it way more amusing that they had the same ARMED "civilian" helicopters in Austria, meaning they must have flown them from Georgia to Austria "over night", which is impossible. These helicopters don't have much range nor speed, would need to do multiple refueling stops and I would like to point out that ARMED FUCKING HELICOPTERS can't just fly around like who cares.

It's so insanely lazy and dumb it's insulting - but most who watch garbage like that don't have the brain capacity to worry about it.