Hi, Deanna,
I have to say that I was a bit worried about your not having enough rest before heading off to your B&B, but it looks as though you have that sorted out.
The Arnold Clark location where you'll be picking up your car is smack dab in the middle of Aberdeen. I've driven in Aberdeen at least 35 times over the years, so I'm familiar with the area. If you have SatNav arranged for the car, it will probably send you to Banchory by way of Holburn Street and Great Western Road. Both are currently a mess due to roadworks, and you can't turn right on to Great Western Road. There are diversion signs everywhere.
The easiest way to get to Banchory is to head north on Powis Place (A96). You may have to go down to the Mounthooly Roundabout to turn about, depending on traffic. You'd make a complete u-turn at the roundabout. Follow the road up to the top, where there's a traffic light. Powis Place becomes Great Northern Road (still A96). Go straight at the next roundabout (Kittybrewster Roundabout), and continue on until you get to North Anderson Drive, which will be signposted for the A92. There is a KFC on the left corner.
You will have reached the Haudagain Roundabout, which, until the bypass was built, was considered the worst roundabout in Britain. Turn left on to North Anderson Drive, and follow it through several roundabouts for about three miles until you come to the A93 (Great Western Road), which should be signposted Deeside. There is a traffic light at that intersection, which unfortunately has no right turn arrow.
Turn right on the A93, and follow it to Banchory, where you can pick up the A980 for Lumphanan.
It's going to take a good hour from the time you pick up the car until you reach Banchory. That's going to put you there around 4:00 p.m., if all goes well. As there's not much in the way of food in Lumphanan, I'd recommend finding something to eat in Banchory, especially as you're planning to get to bed fairly early. At that time, you should have plenty of choices in Banchory, but if all else fails, you can get a "meal deal" at either Tesco or Morrison's (Sandwich, drink, and snack for 4 pounds). Again, if you have time, you could drive down to the Falls of Feugh. If you decide to park in Banchory, there is a parking fee in the main car park. However, most of the side streets have free parking. Just don't park on the dreaded double yellow lines! If you need to stretch your legs, Banchory is a great little town for strolling and window shopping.
Once again, very best wishes, and enjoy the Gathering!
Mike (Auchterless)
p.s.: If you don't fancy tackling the roundabouts on North Anderson Drive, you could continue on the A96 until just past the airport, where you would head south on the A90 Aberdeen Bypass, which you'd follow until you reach the A93, which is signposted for Peterculter and Braemar.
p.p.s.: This close to the Gathering, I doubt whether you'll find any two night stays closer to Braemar. Your best bet at this point would be to go with your original plan, get a good night's sleep on the Friday, and get an early start on Saturday. Leaving the Gathering early would be a good way to beat the bulk of the traffic, but you may be having such a good time that you don't want to leave early :).