This conversation has certainly challenged me to examine this reg in detail. And I think we're going to have to agree to disagree.
.399(b) has no baring on this topic of conversation because it only deals with .169 certificated aircraft.
(a) however, the topic of this discussion is where the accel/stop question comes into play.
My conclusion is this, barons and other small aircraft both single and multi non-turbine aircraft are used. So I am certain they are covered somewhere within 135 as to how much weight they are allowed to operate with.
One part I was looking at was 135.397 although that is for small transport category aircraft. My only question on this topic is what is the definition of transport vs non-transport. I've tried searches in both the FAR book and online and haven't turned up anything I would call a definition.
If the definition for transport category aircraft is an aircraft that is certified under part 25, then I would conclude that part 135.399 has to cover non-transport category aircraft or aircraft certified under part 23.