No idea about transfer costs if booked on Traveloka.
If you take a Blue Bird taxi from the airport to your hotel it might cost around Rpk - Rpk as long as it isn't in the rush hour traffic.
The journey takes about 3 hours and 15 minutes. Tickets will cost around Rpk and you just input the booking number on your booking and the machine at the train station ill print out the tickets. I recommend booking seats on the Executive train rather than the Economy train. We use them all the time to buy train tickets between Bandung and Jakarta and sometimes for hotel bookings. For the train tickets, yes Traveloka and also Ticket. Do you recommend it or should I rent Bluestar Taxi at the airport and buy Bandung tickets after getting to the hotel? We also will travel to Bandung on May 6 returning to Jakarta on May 7.