First and foremost, giving ND the ability to play for the ACC Championship is a non-starter. The ACC should just not allow it - have some self-respect.
It's fairly obvious that ND came to the ACC when the Big 10 and Pac 12 announced they were playing conference only schedules. I'm sure ND was afraid the AAC (Navy), MAC (Western Michigan), SEC (Arkansas), and even the ACC might do the same thing. The fact is ND still has 9 teams on their schedule currently. By forming an agreement with the ACC, they can ensure they will play a full season even if these other conferences decide to go "conference only" in the next few weeks.
The snag comes in that 3 of the ACC's 4 teams that have out of conference SEC rival games play ND this year (Louisville, GT, and Clemson). So those 3 ACC schools would like to play 2 OOC games (SEC rival and ND). To me, the ACC should go with an 8 + 2 model. Negotiate with ND to add 1, 2, 3, or 4 ACC teams to their schedule for this year - whichever number they like. Just treat them as OOC games. This gives ND the ability to guarantee themselves a full schedule and the flexibility to keep Navy and/or Arkansas on their schedule if they so choose. At the same time, it allows the ACC to not cheapen its conference membership by allowing ND to play for a title for one season only.
This would still be an excellent example of goodwill the ACC could use to hopefully get ND to sign on as a full-time conference member while still not giving the ACC the indignity of letting ND use the conference for a championship for one year. It would be the equivalent of the walk of shame for a girl after a one night stand.