Henra: A partial answer to your Q:
The fenstron blades on a 135 are irregularly placed around the hub. They are not equally spaced. For the very reason you mentioned - to avoid dirty vortices off the blade in front. I believe if my mind serves me well they are also configured along two tip path planes (similar to the Apache et al).
In answer to a previous enquiry about Gazelle fenestron stall: The EC135 [it being a mix of the German BO105 and the French Aerospatialle]. The phenomenon experienced by some Gazelle drivers was machined out when it came to the 135 by champfering the edge of the fenestron ring thus widening the "acceptance angle" of the fenestron and reducing (not fully eliminating) the risk of 'fenestron stall'. Technically the 135 can still suffer from the effects of FS due largely to the huge shroud masking the incoming airflow and hindering its throughput. Very few cases (if any) have been reported though.
Finally - TRE. The Main rotor has only to lose a small amount of its energy for the TR to lose a major portion of its effectiveness. We have the use of Westland data to prove this when we simulate decaying Nr scenarios or tail rotor malfunctions. The computer displays the decaying effectiveness as it happens, so, say for EG the nr drops 5%, the TRE drops 20%. There is a disproportionate and large loss of effectiveness from the tail when the MRB's slow even a little. Hence the beauty of controlling Nr with throttles to keep the a/c straight during a TR malfunction.