About conducting gibushim for the other infantry units like the paratroopers
In my opinion, these short gibushim are a very poor test for determining who is more fit for service in an infantry unit and who will still be serving as a combat soldier after 1,2 or 20 years
Fact is, the paratroopers do not perform better than the other three infantry brigade units
I think that for the paratroopers, the gibush--and the parachuting-- has remained more for sake of tradition /prestige than for any proven utility
About selecting soldiers for the elite brigade companies with a gibush at the end of tironut rather than at the start
I agree that your method would be much better for selecting the best soldiers -- and would would also enable having a report of the soldiers actual performance during these months
I may be wrong, but I think that such a method was used sometimes in the distant past (before my time). I read that in the US Army, the elite Rangers are selected from soldiers that have first served in the airborne infantry units
My guess why they don't use this method, is that the separate companies prefer to begin molding the candidate to their special needs from the beginning, as well as to preserve a sense of separation and superiority from the soldiers of the regular battalions. If the sayeret soldiers originate from the same pool as the battalion soldiers, they might discover that they are not that elite
Soldiers that I met that had originated in sayarot and were later reassigned to battalions always seemed to be surprised that the battalion soldiers were not as inferior as they had previously thought and the work in the battalion much more difficult than they had imagined