Is it even possible to have level boarding on the legacy streetcar network? I get the impression that the swing out doors on the Flexitys jut out just enough so that there'd be a pretty large gap.
No. The suspension does not maintain a constant floor height like subway trains do (and the CLRV/ALRV did). It is better to have a full step up to enter than be slightly off in height as the latter creates a tripping hazard. Neither option meets accessibility requirements for wheeled mobility aids anyway.
Accounting for that, is it possible to deploy the wheelchair ramp horizontally only rather than also vertically?
No. Unfortunately there is a narrow tolerance for the vertical height of plarforms for the mini ramp, and that height is standard curb height, not the height of the floor.
Our best bet for the time being is to build as many curb-height platforms as possible. Then maybe the next generation of streetcars will be specified to maintain a constant floor height rather than the lowest possible floor height.
For example Queens Quay station has a ground level platform, requiring the full ramp. Raising it to curb height would then only require the mini ramp, which is faster to deploy.