You seem to have not understood what the forward and backward hold actually are.
For starters, they need to be more than 256 units apart.
The actual definition is based on routing - each hold has a number of routes that enter into it.
The point at where you can push forward, and successfully watch all the routes that go into the backward hold with minimal rotate time is called the "forward hold".
This is true of Snowycoast B as well. The balcony route is real trouble for the backward hold - so, the forward hold is the point where you can watch the entrance to that route and prevent BLU from ever getting there.
Here's a diagram:
It is, of course, a distinctly
tough backward hold. All the BLU routes converge into a very small area, so RED has very small rotate time to watch each one. But you may notice that the newly proposed area doesn't actually change that at all, dumping BLU into the exact same 512-unit strip RED is holding.
Saying that the "value of the new flank is to pressure the forward hold without exposing yourself to the backline holds" is mystifying to me, since even as you've drawn it, the backline holds both have direct sight of the new entrance.
And, of course, as I've drawn it, the new route totally skips the forward hold. The entire purpose of the forward hold is essentially to prevent BLU from magically appearing at the backward hold with an uber and taking the point without anything RED could have really done about it, so letting BLU skip the forward hold would be a big "oopsie".
I don't like to describe
basic TF gameplay when I've done it so many times already, but here's how Valve words it:
[Jakob Jungels] Invulnerability also sharpens pacing by helping a team push through a defensive stalemate, when the enemy team turtles up and refuses to come out of their base. With a lack of defensive skirmishers, it's easier for the offense to build up the invulnerability charge without interruption.
Essentially, the purpose of the Ubercharge is to prevent singularly strong backward holds from making a map impossible to capture.
But, if every hold could be broken by a single Ubercharge, the gameplay wouldn't be very engaging. Defenders would feel cheated, because no matter how skillful they were, they wouldn't be able to defend the point for long.
That's why we have the forward hold - defenders have this constant time-pressure of needing to push forward so they have the forward hold, so they won't just lose the point because the attacker medic right-clicked once. If they fail, and the medic uses an Ubercharge on the backward hold, they almost certainly lose the point. So, if RED loses the point to an Ubercharge, it's their point for being too passive, and if BLU doesn't ever capture the point, it's their fault for letting RED retake the forward hold too many times.