Wow... a little late to the party but what an interesting read.
OP- I'm glad it seems like your daughter and coaches have a plan in place for next year. Also, found your thoughts about wishing to have made a slower plan so she didn't get to level 10 and get bored so fast interesting.
This 6.5 hrs thing seems to be a hot topic and has me interested. I think it all depends on what counts as gymnastics training. I do wonder if an experienced hard working level 10 was in the gym by themselves (no waiting around) could maintain skills. I do wonder if you did 30mins on each apparatus (run through routine, do 3-5 reps of sloppy or missed skills and then another run through of routine). You might also be able to steal some extra time from vault if you were just maintaining one vault. You could do this three times a week for a total of 6hrs. You would have to add half an hour each day for warm ups, stretching which would put you at 7.5hrs.
I think the bigger challenge than actually maintaining the skills is developing a fitness program to be able to do the skills and not losing any muscle mass. I had some friends that used to do gym and switched to cheer. One day at the gym they decided to practice their old routines: floor no problem, vault no problem, beam a little wobbly but not a big deal. Bars - a grand mess! No one had the upper body strength left to do hardly any of a routine. Flexibility would be the same you'd have to work this in as well in order not to lose it. This could be done outside of gym time either in combination with another sport, crossfit or independently with advice from coach or personal trainer.
This is all hypothetical I'm not sure I'd head out and try it but if I had a kid interested in something in addition to gym I might try it and see. I also wonder if a smaller training schedule could be used to maintain through school year or for a couple of months during another sport season with increased hours in summer and school holidays