The last suggestion, using a 14tooth 10 speed top cog with 11 speed cogs for the rest of the cassette maybe possible.
From the tech doc on the dura ace 9000 cassettes, you would have to buy a 11-23 as well as a 12-28.
You'd then take the 15, 16 loose cogs, and the 17-18-19 block, and the 21-23 block, from the 11-23.
And from the 12-28, you would take the 25-28 block.
Mash em all together with a 10speed 14t top cog, and you have a 14-28 speed cluster.
Note however... I have no idea if you can put the 25-28 block behind the 21-23 block and have the cog spacing correct... So unless you have a local bike store to try this with, i'd suggest another solution.
With a 7m rollout limit, you can get there with a 36x11 (approx 6.87m). So you could probably just use a blocked off compact 52-36 9000 crank. Imho, this is a better solution, and probably the one that will be used by the majority of those who have migrated to 11 speed this year in the u/17 division.
It's also good because you never need to worry about throwing a chain
And if you use a 11-25 block, you have gears for pretty much any hill that they race over.
As for junior gearing with 7.93m limit... Man, they really don't know how to set gear limits :/ It's a royal pain to make anything in 11 speed clusters match the rollout closely.
You have 41x11 and 45x12... lol
Neither of which will ever be made my shimano, and I doubt anyone will build an aftermarket chainrings for 9000 anytime soon.
Maybe a few are already trying 49x13 with a blocked off 12 cog with 20mm tyres (which should go under by a few cm). i.e. they would be using a 10 speed crank instead of a 9000 crank.
Imho though, your better off with 10 speed for now as you can get everything working with the junior 10 speed cassette range, including running a 52 chainring with a 14-xx block for u/19 level racing.
/\ I raced u/17 AUS level back in my day