When Shelby and Beth change the 'contact' cell phone numbers in Mac and Kelly's phones, the phone screens show different times despite happening simultaneously., When Beth changes the number on Mac's phone, the time is 8:05 p.m. Then when Shelby changes the number on Kelly's phone, the time is 10:30 a.m.
When Mac is fleeing the sorority house in his car and the sorority girl wakes up in the back of his car causing him to slam on his breaks ejecting her through the windshield of his car shattering it, where she shouts she is on painkiller. In the next scene Macs wife is hosing down the girls who are on his unbroken windshield.
It is extremely unlikely a home seller would accept a purchase offer from Mac and Kelly without a contingency in the contract that stipulated that Mac and Kelly had to sell their current home first before being able to close on their new home. Lending institutions also don't allow this scenario to occur except in cases of high household net worth.
After the sorority girls change Mac and Kelly's phone numbers in the other's phones, when the adults are texting "each other" it still shows the history of their texts. The phone would think it was a new number and the text history wouldn't show on the phone screen.