I'll just leave my thoughts on the PD part. I do agree with you on the fact that during transportation, breaking people out is allowed and possible. However not while they're in a hospital or PD or any safe zone, as you mentioned.
As for RPing breaking out the suspect. You were a patrol cadet for quite some time yourself. You responded to multiple calls and got into many situations. What options does a cop have to call for backup or ask for help when someone is pointing a gun at them? The way I see it, is a cop has 2 options (that both will result in either a) being shot, or b) banned for NVL. The first option is to pull your priority button. This immediately alerts all units. Great, but the crim will hear that and you will get shot and banned for not valuing your life. The second option is to try to go over the radio and say that you're in trouble. There is an animation when talking on the radio that the crim will see, that will also get you shot and probably banned.
With that in mind, I simply don't see how a cop can do anything low-key when having a gun to their head during a transport, without it being NVL. Even pulling a phone out with a gun to you is unacceptable, let alone going on radio, making a /911 call, or pulling your priority.
With that in mind, I was curious as to how you (writer of this post), would RP calling for backup while you're getting held up, or how you would go about the situation. To me it seems that the cop getting held up is quite limited on options but if you have any ideas that won't break server rules I'd be glad to hear them!
Thanks for making the post and taking the time to read my reply