'lo and welcome to tasvideos, hope you enjoy your stay. I'm glad to see more people giving Castlevania some lifeblood. I once did a "no subweapons, no damage" run, was rejected for poor game choice (and, upon reflection, could have been improved by about 10 - 20 seconds).
Regarding your questions:
1.In read-only mode, if you load a savestate, it plays back the movie up to where you stopped recording. If the savestate you're trying to load doesn't belong to the movie or is from after your current progress (which can happen if you loaded an earlier savestate beforehand), it brings up an error.
In contrast, you can load any state that belongs to the movie in Read+Write mode and resume TASing. This is how you got your progress back.
I recommend learning the hotkey to switch between the two, as it is really convenient to be able to switch between them quickly.
2.Oh please, do go for it. The Castlevania tricks page is horribly empty at the moment. To edit the page, you can just ask the staff to make you an editor. There's also the
Castlevania thread on the forum in which you can post findings.
I hope that's helpful to you.