Great points, which brings up another theory: Jackie's upcoming wedding!
Sam and Jackie have been discussing the invitation list for their stateside wedding and must've realized that if Nate should be invited....
- They won't be able to afford the bar tab
- They won't be able to afford sufficient meal quantity
- Photos would be ruined
- Too much unnecessary giggling during the ceremony
- Guests would never want to associate with them ever again
All of these factors would have a negative impact on what's supposed to be the happiest day of Sam's life (and 4th happiest day so far of Jackie's life). Either Nate inquired and they had a blow up about it, or Jackie and Sam haven't told him yet but are creating distance now to soften the blow.