You'll be fine. I've done this loads of times. Just have a printout of your return/onward ticket for all passengers ready if asked. It's likely JL will ask at checkin; they are the only airline I fly that routinely asks if I have an onward ticket if not booked on the same reservation. It's unlikely that you will be asked for it at immigration but in principle they could.

When filling out the form at arrival, just put down your best guess for how long you will stay. It's ok if it changes later. They are mainly looking to confirm that it's < 90 days and matches any evidence of onward journey (if they even asked, which they likely won't).