When I started work on this project was when we thought Silksong was gonna release for sure in 3 months (after that XBox Direct thing which claimed everything shown off would release "in this year" and Team Cherry commented on it with "that's what they said" when people asked if that implied a release date), I figured I needed to rush it out before that date or nobody would play it because everybody would go for the real deal... well, with the benefit of hindsight you can tell I didn't need to worry :P
I'm kinda unhappy with the end result due to all the corners I cut first for the jam deadline, then for the Silksong release date: areas are basically 1:1 references to Hollow Knight, every unlock giving you TWO new abilities means they're spread super thin, spells were a last second addition that's a nightmare to balance because there's a lot of damage variation depending on how iframes and hitboxes line up, and the map system also was an afterthought that makes it really annoying to add new rooms. (The map on the pause screen is a photo of my planning papers with a free parchment texture slapped on and originally it didn't show your position, I've manually added pixel coordinates for every room to allow that lol)
I've always been a fan of adding secrets, it was fun hiding basically half the game just to blow people's minds! (The cutoff point is where the original jam scope ended so when I realized I had some time to wrap the game up and improve it before Silksong was set to release, I basically added the second half in a vacuum and stapled it onto the original endpoint). I can't decide if my favorite secret is the five recursively more hidden treasure chests or the super-secret peek into Lake Suwa.
You've pretty much figured the story out, the last thing you missed is that the Mask of Lost Emotion was used to "turn off" Sakura's emotions (to stabilize her) but it worked too well and basically removed her entire personality as well so she's just a hollow shell. Or "hollow heart" for a title drop, I guess
The soundfonts I'm using is like 90% the default stuff that comes with Mixcraft 9, so mostly the Acoustica Instruments General MIDI VST. Retro samples are mostly my own hand-drawn waveforms from the YaruChipFont (which is a free itchio asset I've uploaded here as well, btw). Most of the magic is due to slapping on the right effects (compared to LMMS and Synthfont this is like 100 times easier in Mixcraft so I have like 5 reverbs and delays on everything just because I can :P)