If a thermostatic valve is in the heater loop, the manifold won't get any water until the heater does, so it needs a separate bypass line to avoid this.
The highest manifold temperature will be obtained by directing all water for the heater into the manifold first, then to the heater.
The next (lower temperature) by using the return line from the heater, since some heat is already lost to the interior.
A "T" in the heater line will reduce the heating effect of the water by splitting the water between the 2, proportionate to the relative areas of the fittings. Example: a 5/8" fitting on both legs is 50/50, a 5/8" to the heater and a 1/2" to the manifold is about 3-2 ratio, etc.
