I believe the ability to manage a booking (low level like selecting seats and special meals) is airline-dependent, and for certain airlines it's possible to do so only a certain number of hours before/up to departure. To do so, you'll need the PNR or ticket number at the very least, which should appear on Ctrip after a booking is confirmed and the ticket, issued.

For flight changes and cancellations for tickets bought through agents like Ctrip, airlines will typically refer you to them to do so.

I've used Ctrip for domestic and international plane and train tix, airport transfers (including from overseas airports) and hotel bookings, and can only say that they have pretty excellent customer service. On two occasions when things didn't work out (hotel room booked wasn't available and airport pick up didn't turn up on time), I was given compensation in cash, after simply contacting their customer support number.