It's not true that the entire world forgets reality as it was before the pact. Only those directly affected by it (not counting the pactbound) have their memories altered.
The demon and the human both remember reality as it
“really” is, but other directly-affected parties simply remember
the demon as having always been involved in their lives with no
recollection of the person she replaced.
In this case, that would only include the (dead) wife and son. So unless their ghosts, or maybe the spirit personification of the house itself, are spoken to at some point, effectively no one will have had their memories altered by the pact.
As the example given in the book demonstrates, objective evidence is altered, or at least some of the most obvious evidence is, depending. Prominent family photos on the walls of the house now show the demon in them instead, and family albums in the attic might or might not also be edited. Analogous to how personal relationships are rewritten, no new house sale has officially occurred, but the demon's name is now on the deed to the house anyway: the pactbound appears to have never owned it at all, and records show the previous owner instead sold it to the demon. (Said previous owner has probably never met the demon in his life, and is sure he sold it to the pactbound.)
So friends, neighbors, and any other third parties, who weren't named in the pact as being transferred, are completely unaltered, leaving them to assume "Huh, I guess he just sold his house to this guy. Didn't even put up a For Sale sign." But investigating official records would show a contradiction with their memories.
When the deal is struck, the demon’s Cover
absorbs that relationship – as far as the girlfriend is concerned,
she’s been dating the demon all along. Obvious signs of their
relationship are likewise altered, such as prominently displayed
photos of the couple. Depending on the demon’s Cover rating,
things like the old photo albums in the hall closet or the ticket
stubs from the play they saw on their first date might not be.
Likewise, the girlfriend’s family and friends remember her
dating the pact-making (now ex-) boyfriend, not the demon.
Depending on the nature of the relationship and the people
in question, this might get blown off (“Huh, I guess things
didn’t work out with Mark”) or raise serious alarms (“You guys
were getting married in March! What do you mean you don’t
And a pact benefit of Cover is a completely separate option from the pact benefit of a Soul, there's definitely no mechanical need to combine them. There's no limit to how much Cover a mortal can give from his life, even if it takes multiple pacts to wring it out of him.