I don't know why some people think you need to sequence break to make any progress after beating Nazrin: you're supposed to go fight Ichirin to get the underwater swimming upgrade, which lets you then go through the waters in Murasa's area and let you fight her, who gives you the ability to lift heavy blocks, and THEN you're supposed to get the Boots upgrade through the passage that opened up to you in Ichirin's area.
Indeed, getting the Boots right after Nazrin sends you down the completely wrong path, making you believe the Magic Bomb is the item to get afterwards...which involves you going through two whole ass new areas without a single boss fight. Isn't that just weird from a design standpoint?
Regardless, after beating Murasa, you're intended to go fight Shou next, who gives you the air dash, then Nue, who basically gives you the upgrade to that (+1 air dash and the ability to do it upward). But,
the game's really open ended by design, so it's not that surprising lots of people ended up thinking sequence breaking to get the boots early was the intended solution.
In fact, I gotta say, this game's so
much more open than I expected: in my review, I wrote that the game is only "wide open" after you get the double jump, but recently I found out it's possible to skip that and by extension Nazrin too: in the hall where you think you need to double jump to get out of her area, you can actually just wait for a koakuma/"medusa head" to come by and jump on it.
After that, it's possible to then use the Bike + the Pink Metroids in the area with the Boots to supplement your lack of a double jump there to get them early.
And from that point, getting the Magic Bomb should be trivial if you already know your way to it, even without the double jump.
And with those two items(the boots and the bomb),
it's fully possible to beat the game without encountering a single one of Byakuren's disciples, through careful use of the bomb jump + hopping off enemies in key segments. Getting the "bike summon" accessory is necessary to beat the game under these restrictions though.
The reason I mention all of this is that, the game's easy difficulty and numerous platforms make so much more sense when you're going through them with so few movement options: the difficulty seems to largely account the player going through the areas with minimum upgrades and level ups.
It's too bad that, besides some dialogue changes regarding Byakuren being completely unaware of her disciples' presence in the castle, there's no special porn for doing all of this...well, I guess the Ending 5 variant with Nazrin could be considered the special secret scene for performing all of these sequence breaks, but that's kind of lackluster.
Now, the only thing I'm wondering is if there's an Ending 5 variant for not having fought the Succubus in the flesh tentacle area...I think the final area is unfortunately completely cutoff until you beat her though.
EDIT: Yup, it's possible to skip the first fight against the Succubus (by using the bike summon to activate the switch meant for the power bead item). However, doing so just completely removes it from your file as if you had already done it, so there's no ending for it.