Other than the downwash effect, what other factors that affect the flow to the horizontal stabilizer? such as Dynamic Pressure? Air Flow Disturbance?
The answer depends on the relative position of both. If the wake from the wing directly hits the elevator, its reduced dynamic pressure will reduce elevator effectiveness.
Next, when near the stall angle of attack, the wing root will already exhibit some flow separation, and the turbulence of that flow will show up in buffeting on the tail. This is sometimes desired as a stall warning, but having the tail move into the wing wake just when the aircraft stalls is generally a bad idea.
To address the lift-related vortices: Those will reduce the angle of attack change on the tail relative to those on the wing. The increased strength of the downwash speed with lower flight speed (and, consequently, higher angle of attack) means that the effective angle of attack on the tail will be reduced. This narrows the range of angles of attack in which the tail needs to work and helps to ensure that the tail will show a linear response to changes in the flow even when the wing leaves the linear range already (in other words: When the wing stalls.)