Reality is a bitch. I perfectly understand why you are going to Pearce Ferry instead of taking out at Diamond Creek--just too darn expensive to take out at Diamond. But it does take two more days to get from Diamond to Pearce.
What we did when we had some 2-weekers along was to use our 14 days between Lees Ferry and Diamond Creek, and then let the 2-weekers catch the Indian bus up from Diamond. The Hualapis run their motor trips from Diamond Creek every day, and the busses that carry their passengers down to the river go back up empty. We managed to find someone who could sell us space on the bus (I think they charged us $50 per seat). The 2-weekers got out on schedule, and the rest of us spent three liesurely days going to South Cove.
The bus leaves Diamond Creek at about 10 am, and they will not wait for you, so if you have people going up, make sure you get there early. Also, be sure to take your receipt with you, because the drivers don't always get the information that they are supposed to take passengers up the hill--something that they rarely do.
Below Diamond, the name on the permit is of no interest to anyone, so you don't need to worry about that. The only issue would be whether you have enough rowers left to get the rafts to Pearce Ferry.
Just one more idea. I'm sure you can still have a great trip, even on the 14-day schedule, as long as you set a daily mileage goal and stick with it. But you'll have to sacrifice some off-river sightseeing.