I can maybe provide a few answers here. I was involved in steep approach certification on both the BAe146 /RJ and the Jetstream 41.

The length of the runway is in itself irrelevant however combined with a steep approach of 5.5° as it is at LCY makes it relevant.

What happened on the certification programmes I was involved in was that initially handling trials were carried out using visual approach aids (not at LCY) at 5.5, 7.5 and 9.5° approach angles The higher angles were required for abuse case, tailwinds etc. The aircraft handling and ability to flare were assessed. Factors such as airbrakes, flaps, thrust (with engine antice on, it puts the idle thrust up), CG position , landing weight were all assessed. Only when all these tests were completed, tests at LCY were flown to assess the aircraft when the autopilot was coupled to the ILS, something that couldn’t be done anywhere else due to the glideslope. At LCY factors such as ground handling, noise etc can be assessed as well.

On the 146, the aircraft has to be set up in the landing configuration before descending on the glideslope otherwise it will not be able to slow down during the approach. There is a steep approach switch in the flight deck. It gives an indication of when the aircraft is in the correct configuration for the approach and desensitizes the GPWS Mode 1 “Sink Rate” call out. The Jetstream 41 steep approach switch did something similar.
The approaches use standard VREF values so are no faster or any more hazardous than normal approaches
The flare is awkward as the aircraft has to be rotated through a larger angle than during a normal 3° approach and if the flare is not correct, the aircraft may float – that is when the short runway becomes a factor. LCY has touch down lights set so that if you are not on the ground by these lights then you go-around for another attempt.

I assume Embraer will have gone through a similar test process and I believe the aircraft has had to be modified to allow steep approach certification.

I hope this helps clarify a few points.