1. I notice a plus sign after a preset name which I can't rename away. WhatMeans the preset noted has been applied, but manually modified by you. Dbl clicking on the preset usually clears any manual deviations from the preset image and brings it back to the preset’s original settings
does it mean? How did it get there?
Presets are exclusive. Changing from one preset to the next cancels out the previous and applies the current.
2. Are presets additive, mutually exclusive or what? ie If I've already
used a preset will using another, different preset overwrite or add to the
first one?
A good resource for questions like these is Martin Evening’s Adobe Lightroom CC.