I feel you're approaching the problem in the wrong mindset. You're imagining that you have one module and that's all you ever have. Instead, think of a the CSS module (your .testimonial module) as an interface - a blueprint to implement different types of .testimonial module variations.

The BEM architecture calls these "Modifiers" (http://csswizardry.com/2013/01/mindbemding-getting-your-head...).

When I look at your example, I don't think of a .testimonial object in two different contexts (one vertical in the sidebar of the signup page while one horizontal on the pricing page). Instead, I think of two different variations of the .testimonial object. So while your .testimonial class may define certain global styles, you change what you need by variations like .testimonial.vertical or .testimonial.horizontal (though I wouldn't actually use those names, but you get the idea).

This requires making a decision on when to apply what classes, but that's okay. You shouldn't be trying to abstract away actual UI decision-making to an automated tool or process. You seem to think that settings rules and logic to be executed based on those rules (when x, execute y; when y, execute z; etc.) will mean you always end up with a usable interface. However, this is not the case.

Ultimately, UI is not "write-once, use anywhere", and that's okay.