Sanny - we went up Bynack Mor from the Ryvoan side as day two of a three dayer in May, heading towards Braemar. It was pretty wet although the bottom half was rideable. Thereafter it was a push as it's pretty rocky, pretty solid rocks sticking out of peat and gravel. As it was so wet our rear wheels just sunk into the peat and we had 50 miles in the legs from the day before and quite a lot of beer so it made sense to push to the top
I would imagine much of it would be just about do-able in the dry and with fresh legs!
There's a couple of nice descents from the top, not too long and not too tech and then it's quite steady to the Fords of Avon, heading south. Again, because it was so wet this bit was hard with lots of on and off, ride 10 yards, walk 10 etc. Once at the Fords of Avon there's some more pushing and riding to get to the top of the Lairig an Laoigh but eh descent from then on and the trail toward Derry Lodge is great.
I've ridden down Bynack Mor to Ryvoan and it's a great descent, multiple line choice fun as the track is wide. I did it as part of a loop from Tomintoul (south to Inchrory, up Glen Avon to the Fords of Avon and then the opposite of the way I described above, Ryvoan Bothy, Dorback Lodge, Glen Brown and back to Tomintoul).
I guess an alternative route would be to do Bynack Mor (either way) from Glenmore as a loop via Glen Feshie and Glen Geldie ot the Lairig Ghru. I think stuartie_c has done the Larigh Ghru option.
I can't comment on Lochnagar (it's on the list!) or Ben Macdui.