That is, although PS was no longer stopping me from using it, I couldn't seem to make any change!
Could you post another screenshot of a situation where this happens (with the Brush Tool selected and Layers and Channels Panel and the Options Bar visible)?
There might be a couple of possible reasons why painting seems to have no effect even though the Layer Mask is selected.
Thank you for your honesty....
Well, generally honesty seems to offer some advantages over the alternative …