I don’t know if this’ll work, but what if we do Lakitu Skip, then go into the castle, blj through the Star door, skipping a Bowser fight. I have not played the game, only watched speed runs/TAS and I’m wondering if this’ll work. Feel free to give your thoughts and suggestions on this topic!

I don't know which star door you mean, but I can assure you it is almost certainly not possible to shave off another key for humans.

Also, you can't just BLJ through whatever you want. You need to have some sort of ceiling or slope so that you can increase your speed quick enough. Doing it on a flat surface just won't work.

You can LBLJ through the door in front of the stairs is what you mean, but you won’t be able to skip the first bowser due to animation

The upstairs of the castle doesn't load unless you open the door with the key from Bowser in the Fire Sea. Just clipping through that door isn't enough. You need to do Bowser in the Fire Sea, which means you need to get to basement somehow, and the only viable way for humans to do that is with the key from Bowser in the Dark World.