I'm still scratching my head about the part where they had to (literally) throw their 5C mouthpiece away for "personal reasons."
So many of these threads remind me of the story told by the clinician at a percussion clinic I'd attended. The clinician talked about going to his instructor right after he got to college seeking advice for how to improve his drum roll, ostensibly to gain knowledge of the trick or tidbit of wisdom that was going to switch on the lightbulb and give him a perfect drum roll. His instructor simply said, "If you want to improve your roll, then roll - 10 minutes a day, every day."
Translation: most often the answer is focused practice on the area that needs to be improved.
In my own playing efforts and practice over the years, the issues and solutions to most of the basic issues have been self-evident:
Articulation is too hard and rough - work single tonguing and attacks.
Sound needs improvement - long tones!
Can't double/triple tongue cleanly/clearly - work on double and triple tonguing.
Having issues with lip slurs - do more lip slurs.
Having issues with a technical passage - slow things down and work through the passage, over and over and over until it's under the fingers, then speed up to past the necessary performance speed.
Playing trumpet effectively isn't rocket science and many times improvement can be made, simply by spending the time in focused practice for what needs to be improved. At the end of the day, playing trumpet is a matter of combining some pretty basic elements in order to make music -
Articulation with finger/tongue coordination
Range can be an issue, but if you work your sound to the point where you have a solid, full sound, you should end up with a 2.5 octave range - low F# to 2nd ledger C - and the largest part of trumpet music is written right in that range. It has been my observation that those who have range issues typically have basic tone production issues as well.