How does bank angle differ from roll angle with respect to stability in fixed-wing aircraft?
In my experience, "bank angle" and "roll angle" mean exactly the same thing. I've only ever heard of one meaning of either term. 

For completeness, here are some "plain English" definitions of all three Euler angles:

Pitch is the angle that the nose (the longitudinal axis) forms with the horizon. If the nose is pointed at the horizon, the pitch is 0°; if it's pointed straight up, the pitch is 90°; and if it's pointed straight down, the pitch is -90°.

Heading (sometimes called "yaw angle") is the horizontal direction that the nose is pointing, measured clockwise from true north.

Bank angle (sometimes called "roll angle") is the amount that the aircraft would have to roll (rotate around its longitudinal axis) in order to bring the wings level (which is to say, bring the lateral axis parallel with the horizon), with the top side of the aircraft facing above rather than below the horizon.

Pitch ranges from -90 to 90. The other two angles have a range consisting of a full circle; I think bank is usually expressed as a number from -180 to 180, and heading as a number from 0 to 360.