The definition of Va in Part 23 is clear. It has nothing to do with
control surfaces and everything to do with stall speed and load
factor.

Then you haven't read Part 23.

Let me point out the sections to you:

-------------snip------------------
Horizontal Stabilizing and Balancing Surfaces

§ 23.423 Maneuvering loads.
Each horizontal surface...must be designed for the maneuvering loads
imposed by the following conditions:
(a) A sudden movement of the pitching control, at the speed VA...

(b) A sudden aft movement of the pitching control at speeds above
VA...

Vertical Surfaces
§ 23.441 Maneuvering loads.
(a) At speeds up to VA, the vertical surfaces must be designed to
withstand the following conditions....

Ailerons and Special Devices
§ 23.455 Ailerons.
(a) The ailerons must be designed for the loads to which they are
subjected -
....
(i) Sudden maximum displacement of the aileron control at VA. Suitable
allowance may be made for control system deflections.
-------------snip------------------

Now, the section that may be misleading you is this
---------snip-------------
§ 23.335 Design airspeeds.
(c) Design maneuvering speed VA. For VA, the following applies:
(1) VA may not be less than VS * sqrt(n) where -
---------snip-------------


Note that it says MAY NOT BE LESS THAN... In other words, it can be
more.


Oddly enough, many aircraft manuals bear this out, providing lower
Va speeds for lower weights.

Oddly, you didn't read what I wrote.

The point is that at Part 23 doesn't require this. And not all
aircraft publish such variations.

does not mean that the maximum speed at which you can fly
and be assured of not overstressing the airplane does not go down as
weight is reduced.

Again, you didn't read what I wrote. I said it doesn't scale UP.
Flying over max gross may increase maneuvering speed, but it doesn't
increase VA, because the increased weight won't protect control
surfaces from failure.


Even your control surface tangent isn't really relevant to this
particular thread

Tangent? It's the essence of what Va is.


I seriously doubt Todd has told you that Va remains the same
regardless of aircraft weight. You obviously misunderstood him.


Ok, you read what he wrote and tell me:

----------------snip----------------------
Note that this is a minimum Va ("no less than"). Thus the
designer can specify a higher Va and then protect the tail
surfaces by limiting stick throw, or by making the required
force at the stick to produce a damaging load on the
protected structure higher than a standard pilot could
exert. Note also that the regulatory definition of Va
*requires" that it be computed only at a stalling speed "at
the design weight" , i.e. max gross. Thus any lower speed,
or any lower weight cannot be Va as defined.
----------------snip----------------------