rowuk

I don't think it makes any difference. Whistling is efficient when the chaps are fairly relaxed. Your breath support when whistling is also only a fraction of what a trumpet needs. No danger, no benefit other than perhaps an alternate way to relax.
You do not need other instruments to learn how to focus on the trumpet. You just need to focus more while playing (which normally involves not beating yourself up as much). That being said, EVERY additional experience makes you a better musician. I used to think that playing tuba helped my trumpet chops, it was just more time breathing and when I compared results, there was no real advantage, except being able to give the tuba player in our brass quintet a hard time!

__________________
Whenever I feel blue, I start breathing again.