Cl does decrease, but total lift force is constant regardless of airspeed (assuming level flight)
Agreed
This means that to maintain lift at higher airspeeds (lower aoa and Cl), more air mass gets accelerated faster (vertically) giving
an increased downforce on the tail. F=m*a
Except that it doesn't work like that. Force is equal to the rate of change of (vertical) momentum; that is it equals the mass flow over the wing times the vertical velocity imparted to that air by its passage over the wing. [Just rewrite your F=m*a (or F=m*(dV/dt) to read F= (dm/dt)*deltaV - still the same equation but different emphasis]
As you point out, at a higher airspeed that is more mass flow and since the lift is constant that means that the vertical velocity when the air leaves the wing must be less. But that vertical velocity is the downwash ...
If we are just talking about the changes in force on the tail that result in a stable airplane, then yes, lower AoA goes with a bigger tail downforce because that is what pitches the airplane back towards its stable condition - I don't have a problem with that, but it it the change in tail AoA that comes from the reduction in aircraft AoA combined with the change in downwash angle that produces the additional downforce (at constant airspeed that is).
If we are talking about changes to the original trim coming from an increase in speed, then both wing and tail forces are going to be increased and unless the AoA is reduced the aircraft will develop some normal acceleration.