We have used the five minutes early and fifteen minutes late grace periods with no issue.
On our last trip, my son was traveling with his high school marching band. I made FP for his friends "day of," because they didn't get their tickets early. As a result, I had to do lots of overlapping windows. Several times, one of the group of six had a FP that was already past the grace period by the time they arrived, but they politely explained why they were late and asked if the one person could go in with the other five, and the CM allowed it in each case.
The most extreme case was for FoP. We had FoP FP booked for my family (we were staying onsite, but the band was offsite,) and we decided to let the kids use them. We used the change party function and switched them over, but there was one more kid than I had FP for. The night before, I found a FoP FP for 8:15 to 9:15 am. I knew there was zero chance of the band getting there that early, but I booked it in hopes that the CM would be gracious and let the kid with the expired FP go through, which she did, even though it was 10:15 am. FoP and 7DMT would be the riskiest rides to do this on, and I wouldn't recommend it, but it was really my only shot for this kid.
Speaking of FoP, I managed to find two sets of two FP for our arrival night, but there was 25 minutes between the windows. (Five minutes if you counted the grace periods.) We all went at the end of the first window, not during the grace period but the actual window, and asked if the later two might be able to go in early so we could all ride together. We were prepared to be told no and would have been totally okay with waiting, but the CM allowed us all to go on together.