1. Yes, separate forms for each adult.
2. & 5. Yes, you can prepare the forms before leaving UK, but they can only be submitted, and printed off, less than 48 hours before arriving back in UK.
3. Yes, printed copies are acceptable, in fact by all accounts they are preferred.
4. Not sure if the test reference numbers can be entered this early, but even so I don't think the form can be completed/submitted online more than 48 hours before arrival in the UK.
Finally - the UK PLF is for return to the UK and its availability (along with other necessary paperwork) will be checked before boarding in Madeira.