The first observation is that the supercharger being a positive displacement air pump virtually meters the air volume going into the engine. So placing a turbo compressor after the supercharger cannot increase mass flow beyond what the supercharger is displacing.  All it can really do is reduce the discharge pressure at the outlet of the supercharger once the turbo compressor starts developing some serious pressure differential of it's own.  But you are already fully aware of all that.
The second observation is that a standard turbo blow off valve arranged as you have it, will be very violent in operation.  It will either be fully open or fully closed, leading to some fairly dramatic changes in boost pressure upstream of the throttle during driving.
With a slightly open throttle, there will be massive boost pressure developed upstream of the throttle, because with the blowoff valve closed, the supercharger displacement will meter the air volume going into the engine, rather than the throttle position. Think about that for a while...
A sensitive and very progressive air bypass that fully unloads the supercharger at small throttle openings is an absolute necessity with the throttle located upstream of a positive displacement supercharger. A crude turbo blowoff valve is not going to work very well for you at all doing that job.
Thirdly, the additional proposed throttle body air bleed valve ahead of the turbo may also be somewhat problematic and erratic in it's opening and closing, it may even flutter, leading to additional sudden surges in pressure ahead of the throttle body.
The turbo may be very unhappy, and realise that any negative pressure at the turbo compressor inlet is guaranteed draw oil into the compressor housing. Very few turbos these days have positive oil seals on the compressor shafts that can tolerate a vacuum, including the RHB5.
My thoughts are, that this may not feel very nice at all to drive, as the air pressure ahead of the throttle is very likely going to vary wildly with throttle opening, and the operation of the blowoff valve, and throttle body turbo bypass system.
If the turbo and supercharger are already mounted on the engine, some pipework changes should not be too difficult to implement.

MORE NEWS