from my understanding – the Notary is OBLIGED by law to conduct a search prior top the notarial appointment on the day of the appointment (usually one hour beforehand) to ascertain that no outstanding charges exist. Furthermore he will ask the seller to Declare that there are no outstanding charges. Once confirmed if there are you would in anyway have legal recourse (and that is another matter in itself) to claim back any charges that notary didn’t spot (highly unusual but not impossible)
So unless the seller is quick he couldn’t organise another mortgage (he couldn’t anyway as this would have to go through the notary.
Also once the property is signed for in front of the notary it is “Blocked” from the seller or anyone else but you registering the property or debts against it for 10 days. After which providing you are the named person only YOU can then have debts against it.