not sure about flight records
If they do, then concealing you have travelled from the UK within a day or two would be useless.
But, now, if they find out sb lied in that form, what is the real responsibility? They can`t detain you for such banality. Perhaps they might give you a ban on visiting Poland.
For how long on average?
Depends on where the subject lives and how many staff are sick/isolating
Yes, exactly.