Shape Keys & Drivers for both Biceps - help please for odd behavior

I have a fully rigged human model standing in Front view (num 1) with arms stretched out.
If the forearm is flexed, the biceps don’t swell. So I wanted to create this effect by drivers.
Take the left arm. I flex (bend) the forearm and in this position, I inflate the biceps by sculpting. I then add a Shape Key (Biceps.L) and its slider shows value=0.
I right click the slider, it turns pink and I add a Driver and in the N-panel of the Graph Editor I fill in the slots, including forearm as driver and choose Z-rotation and Local space.
I note (with satisfaction) that a swell appears in the biceps in the flexed position and it subsides as the arm stretches out, as required.
So far, so good - except for one thing, when the left biceps is being sculpted, the right biceps also does the same - altho the x-mirror is disabled (in Edit mode).
Question 1: How to prevent the other arm being also sculpted?

Next if I flex (bend) both arms and sculpt one, the other gets sculpted too - although I made sure the x-mirror button is turned off.
Now I have two Shape Keys defined Biceps.L and Biceps.R. But I notice that their sliders, instead of having value = 0 show SLURPH = 0 or more !!
I proceeded with the drivers but the biceps would not inflate.
Question 2: What is Slurph and how did it come about?
I am not keyframing at all. Help would be much appreciated.