I am not sure you can board any train in Poland without a ticket and expect to pay aboard it - I always thought it was some routes only or where there is no ticket office at the station you join from.
You can do it from any station - it depends on the goodwill of the conductor as to whether or not you'll get charged the extra 5zl (as is now).
It's a good idea to bring in the fines for not going straight to the conductor - I completely admit having deliberately boarded at the "wrong" end of a train for very short journeys before.
I don't know what you're supposed to do if you see no conductor at the door or there's no room at the door to stand and wait for a conductor. Whistle?
The conductor will always be in the front carriage - so long as you're close as possible to that point, it's not an issue. If the train is so rammed that you can't get through - well - neither will he ;)
Worth pointing out that these changes only apply to trains operated by Przewozy Regionalne.