you can pay on the train in cash or with a credit card
I wouldn't count on being able to pay with a credit card on the train - it is available on some trains but not all as far as I know
it can be, although it depends on how you want to communicate
grade your language
Ok I'll try to use simple English exclusively.
you can always switch back to normal English once you encounter someone able and willing to communicate that way :)
The taxi situation sounds kind of nasty. Is there some other way to get from the airport to the train station?
there sure are busses that connect the airport with the city centre - use the 175 line - it goes straight to Dworzec Centralny (the central station) - use some tablets or cardboard with 'Dworzec Centralny' written on them to show to the bus driver to buy a ticket - the ticket should be less than 10 PLN (I am not 100 per cent sure though) so if you give him a 10 PLN note it should be fine (you can use another tablet to tell him to keep the change ('RESZTY NIE TRZEBA') - (don't expect bus drivers to know English simply)
EDIT: you should actually listen to hythorn advice: he seems to know the realities - I just used the internet and my imagination