If I had to do over, I would have a single sensor post turbo. For only 1 reason, no other. Fear that the lambda may not like its new elevated temperatures, shooting out metal into the turbine and having the car off the road.

That's my pure mechanical reason, as per your request!

depends on the manifold too

Lets say you have to buy a expensive rothe turbo manifold for r32, would you drill and weld to it? Dont think so.

I would run 1 post turbo.

I deal with lambda sensors at work...

The only pre-turbine sensor i know is the BOSCH LSU adv lambda probe.
All other types BOSCH LSU 4.x and NTK ones are always post turbo.
Main reason is the static and dynamic pressure dependency of the probes.
Here the LSU adv is less sensitive to pressure gradients.
Also the material of the protection tube is more heat resistant.

Further on modern ecus own a pressure and temperature compensation calculation of the O2 signal.
Temperature dependency is also reduced due to closed loop heater controls.
But on ME7 i think you wont find temperature compensation.