There is no evidence of the trainee camping on hospital, which is the only thing possibly a rulebreak in this whole situation (F1 states they can only arrest when passing by coincidentally instead of actively camping the hospital).
Once the anti-arrest timer, giving you 20 seconds to spawn your car and get the hell out of there, was added for hospitals, the "hospital arrest" rule was removed and turned into the camping rule.
Besides that, when he opened fire you had already left the hospital area; even before the hospital arrest rule changed into hospital camping rule, if this would happen back then, it would already no longer be seen as hospital arrest/attacking by the cop.
Conclusion: his claim that you 'had no grounds to stand on' is right in all aspects of this situation. If you only brought forward some evidence that he was camping the hospital, it would be different.. altho I understand that's nearly impossible with the timing and speed of your respawn. Anyone who has dealt with him shortly before could have helped you out and report with additional evidence, so ask for that next time maybe.
