A problem might be that the 18:10 SK flight to CPH is often fully booked (on a B737) whereas the 19:00 is often a A321 with more seats and is less popular for business travellers going back to CPH after a day in Stockholm.

I do not know the rebooking conditions for J class ticket, but if no rebooking is allowed you can forget about changing flights in advance + J class on the 18.05 flight will most likely not be available anymore. Your only hope would then be to ask at the gate at ARN if you can get on the 18.05 flight – I doubt it so don't get your hopes too high. I doubt Turku will be able to do anything if rebookings are not allowed or J class is closed.

You can not assume that the CPH flight will leave from the gate you expect.

If you are rebooked you will still be in J class (or higher). If you do not get EB points for J class just send you BP and E-tickets receipt to EB retro registration in Sweden or BKK.

If you ask me it is really not worth it. Whether you wait at ARN or CPH is hardly important. A 55 min connection at CPH is more than enough in the evening. Most likely the PEK flight will wait for you even in case of delay -most likely there are other pax bound for PEK too on your ARN-CPH flight.