would writing to a name table not currently being rendered cause glitches? or do all writes to vram durring non-vblank time cause errors?
I think they'd be causing errors, since the PPU itself is messing around with the VRAM address while it renders.
Yeah, the register used to access PPU memory (accessed through $2006) is used during rendering. Anything you do with it during rendering will mess the current display and you'll probably not suceed on what you were trying to do.
It doesn't matter which nametable (or pattern table) is being accessed - the fact that one is being accessed at all means you cannot mess with $2006/$2007 during rendering without causing problems.