A well-placed midpoint saves this from being any harder than it need be; a badly-placed frog makes the disco section much harder than it should be. Good thing that the tail-end of that sequence, the third disco shell ride, can be tanked. The radish before the amusing-but-somewhat-unnecessary boss is a godsend - if it were my hands on the level editor, the leek would be where the boss door is right now. Sod rinkas all the way to heckageddon.
The Secret Exit route has a neat gimmick (using time stop during a raft ride) that is mercifully cheeseable with hits, which are an effort to hold onto anyway, so it can hardly be said to be
totally cheesing it. I'm surprised we haven't seen a level to really use time-stops as a primary gimmick throughout, only Lusco Fusco's Lakitu bridge comes to mind. The big box with a radish in it is just (amusing) trolling, though!
I get the physics comment with regard to the shell-bouncing and aerial control, but I just loathe SMBX ice/slippery physics. There's a lot of good in SMBX, but vines, ice, and lava hit-detection are all heavy marks against it that make me wish the whole thing was open-source or at least still under development - here's hoping
kil's interception/extension work leads to something good in that direction? (I have no idea just how feasible/complex physics-level alterations may be with a DLL interception approach).
Nice MAGL reference at the end there, you are right to dread
Jolpe's contest entry!

That said, hats off to the man for Deltinus Castle, it went far better than I feared (even if it did bring out the French sacrilege) and certainly didn't seem to torture raocow the way it did Frozelar.
Only three leeks left, and two actual levels! PS. raocow you can now access Top Secret Area!