Wow. No you would not be rude to insist the trainer does their job. Their job is to train you, individually for half and hour. Full stop. If they are asked to do anything else during that half hour, they need to say "no, I'm sorry, I can't help you right now, I'm in a contracted training session." And if your trainer does not refuse to be called away, you have every right to be refunded the money you spent on the training session.
As for how the gym can handle this - perhaps trainers could wear a certain color t-shirt, while regular staff wears another (perhaps even with a "Ask me for help" message printed on it, or a button that says something like that). This way members know at a glance who is available for general help and who is engaged in training. Also the regular gym staff should be walking the floor regularly checking equipment, available for questions, making sure gym rules and basic safety is being observed, etc.