The most obvious solution to me, if all of that information is accurate, is that there should be a distinct audio stall warning for alternate law.
Instead of the plane saying "Stall... Stall... Stall... " when in alternate law it should probably say "Stall... alternate law... Stall.. alternate law...". Doesn't guarantee you the pilot will listen, but it seems like it is pertinent enough information to push it front and center along with the stall warning.
The problem is that making something more noticeable automatically makes everything else less noticeable...
It does not appear that it's very clear when the plane goes into alternate law & it does not appear they receive much training on how to fly in alternate law. Even if they do receive training, you probably do not want your most novice pilot handling the controls in alternate law. Additionally the averaging of the inputs seems to be the most obtuse way for pilots to realize mismatched inputs.
I think we've discovered the most difficult UX design task on earth.