Just reading through this.
For a basic level comp - I've seen solos started anywhere from 4 months to 4 days before. Depends on the coach, depends on teh skater. Some skaters can pick up choreo quickly, some take longer;some coaches like to teach elements extensively in isolation and then assemble, others like to teach elements in a solo. It depends.
Music choice - we do not have any input, nor have we ever, and my one kid skates at the National level. However, we may suggest a "genre" i.e. that Latin or Jazz might be nice; we might, on occasion, get 2 or 3 to choose between. However, all options are chosen by coach and choreo. As they always have good music, however, we're happy with that.
Coach has complete veto on costumes. Choreo describes what she wants, we work with her and the coach to get it.
Now, within the same rink - other kids pick their own music, their own costumes, etc. It varies coach to coach. But, I do know that what has a competitive differentiator for our kids is that extra "level" that music and outfit have made: this is their expertise, and it pays off. Music is a big part of the impression on the ice.
Sandbagging ... well, that's a hard one. I can list the coaches who sandbag in our area very easily. I can also list the parents who gossip and backbite and spread rumours about sandbagging just as easily !
It's not always "sandbagging" when kids are successful. A coach may hold back a skater who isn't ready in terms of confidence to take a risk at a higher level - particularly for a first comp - as they want the skater to continue to be enthusiastic and eager to compete. I only call "sandbag" when, as Skittl says, if a kid is consistently winning everything over a period of time ... not where they win, then test up to a new level where they again win, and onwards. Some coaches want a kid to always be "hungry" and at the back of the pack ... others want them to be successful and then move on to new success ... different philosophies.