In the first place, (for me at least) it wasn't exactly obvious that it was a shortcut, as opposed to a missable secret. Especially since Juni's stated goal is to
get out of the dragon to kill it. And since "green glow" is shorthand for "secret nearby" rather than "end of level nearby".
Secondly, in my eyes at least, being able to get past that room without the umbrella is a feature. The flint, with its plethora of "detect target"s but no justification for them (end sequence, convenient reason you can't do the end sequence*, alternate end sequence**) is what needs fixing. And you don't need to know how I got there to fix
that. Heck, if you're dead set on ensuring I don't get to the flint before I should, you already established that passages open and close inside the dragon, so just put a floor over where you'd drop to the flint. (Preferably put something there to take the sting out of essentially starting over. Maybe someone got eaten before you did?)
*
Perhaps you could pull the same trick you played with the hologram, and have Juni lose the sword blade but not the hilt as she falls down the dragon's throat. (Maybe coming off as she plunged it into the throat lining) Then you could have the umbrella you find (in dialogue, at least) missing its handle, and insulation (if any) on the tip. Umbrellas are made of metal, right? (I forget whether the sword was pictured in the cutscene or not. I'm also assuming that the hilt is enchanted, and would carry that enchantment to a suitably metal thing.) Presumably, you wouldn't be able to start the chain reaction with just the hilt.
**
#1: end sequence as normal, except instead of going to a new character, you end up in a void, where that guy from Silver Falls yells at you for breaking the universe:
Guy: You were supposed to get the umbrella! and go through the rest of the level! But noooo, you just had to-
Juni: There was an umbrella?
Guy: What did you think all those updrafts were for; scenery?
#2: Because Juni killed the dragon earlier, the dragon falls and is impaled on Jotunn***'s house, and Juni lands in the rejuvenation stew Jotunn just so happened to be making for herself, with the purple shrooms you so helpfully gathered for her. (I assume she's not fibbing about her joints.)
#3: As normal, except the dragon is on a different part of the road, and the knights comment on whatever article of Jotunn's clothing is poking out from under the dragon. (does she have ruby slippers or anything?)
***Did I get her name right this time?