1) Is there a way to board/deboard passengers and crew separately. It looks silly to see cabin crew and pilots deboard on short turn-around flights.
This has been already discussed and answered in many other threads and yes, it's already planned for an update.
2) I see all pax wear summer outfit, whereas it would look somewhat ridiculous to see them in this outfit during freezing Siberian winters. Would it be possible to include this option (winter clothes) in future releases?
Everything is possible..
3) What is considered "normal" passenger density, in my opinion is too slow (it took me 22 min. to load 180 pax into PMDG 737). When I set it to "dense" faster moving pax start to gain on and walk through those that walk slower. I understand that more density means less FPS, but still a faster boarding would look more real, IMHO.
If Normal seems too slow for you, just use Dense. No, there's no way to prevent the more likely collisions of passengers at Dense, without destroying the frame rate and putting the sim to an halt (the program would had to monitor the position of everyone, and prevent collisions).
4) Finally. just a though. Do you think it would ever be possible to implement boarding/deboarding Traffic aircraft? I know this is technically hard, so you should know better.
It's not technically hard. It's too hard on the simulator, because we would had to monitor all AI, and send 20x more the same amount of commands we are sending now, totally disrupting both the sim and the other addons that are running with it, by overflowing it with commands sent to Simconnect. This, without even starting to take into account the possible fps hit of so many more models to draw.
Even with just 10 AI, drawing 10x the number of stairs, 10x the number of service vehicles, 10x the number of luggage and (this is huge), 10x the number of passengers, will bring the sim down to an halt.