In recent times I have noticed more Aeroflot A320s at St Petersburg on the Moscow run. Aeroflot are steadily taking delivery of more of these so don't be surprised if you get one.
The seat backs folding forward is a standard feature of Soviet types. Should the unoccupied ones fall forward on touchdown/braking, as they can tend to do, it is no issue, just put them back up again if you wish. I can't see how you could feel trapped at all. They are done like this because when boarding/disembarking it makes access to the seat row behind easier. Just one of those things where Soviet designers did it differently.
The 154 feels notably larger than a Trident, it's bigger than the Trident 3, higher off the ground, and the rear empennage is notably larger. I think they have been coming into Heathrow for the best part of 35 years now (although all the earlier models are replaced).
The runway at St Petersburg, notably the southern one of the parallel pair which tends to be used by domestic flights, is certainly rough. In fact it's probably the most undulating I know, including any grass fields I have flown into. When international flights were using it in recent times while the northern runway was out of service BA skippers on the London flight used to make a comment about it before departure. It is fun - like a roller coaster. At least you won't have the enjoyment of departing from it in a blizzard as I have done.