Hey, so from your description, it sounds like your stylus emits a Microsoft standard signal (was built for Ms Surface behavior), once you hold the button down it emulates a "eraser tip" (like for styl...
(2/2) If it doesn't work for holding the button while choosing an alternative brush preset, you can go to the keyboard shortcut, and find the keyword "Eraser"; you'll have the possibility to get a keyboard key to switch to this mode (without having to hold it) and then you can from this mode select the brush preset for it.