I'd say it depends how hard you plan to run. If you're just gonna plod round then you shouldn't have much trouble with either if you're well trained, I'd say the 50 would be tougher than 35s though. If you're not well prepared then you're probably better off doing the 50. I did 4 b2b 30s a couple of months ago and that was a doddle compared to what a ~90 on similar terrain would be.
It was the up and down that ruined me when I did this, think I was well trained for the distance but naive about the hills. They're not even that big but pretty steep in places and once the damage is done you're obviously in trouble. If you're able to run 50 and 69 then you've definitely got the distance. Look for marathons with about 2000m+ vertical or if you live somewhere hilly plot your own. A few of those and you'll be in great shape.
I did mine over xmas and it took about 8 weeks before I could run on it again. The first few runs off road were dodgy as anything as the tiniest little knock would give a horrible sharp pain, feeling like all the healing was just undone. If you really have done it properly then don't rush back! I used a wobble board for strengthening but am not totally convinced by it, I think you can get most of the benefit just standing on your toes.
In terms of technique, look a fair way ahead and plot your line, identify bits that may be loose, bits which look like good places to land etc. I've got a long history of snowboarding so this came naturally, not sure if that's the case for everybody or not. If things get really technical then smaller steps helps, that way if anything does go wrong you're not going to have so much weight on your foot. I use my arms for balance quite a lot too.
The ankle injury has made me much more paranoid, if I've got to go through a puddle I can't see the bottom of or somewhere with lots of loose rocks then I'm now walking, the time lost is nothing compared to another prolonged injury.
Do a DIY jobby, it's dead easy on that coast path, you'll then be able to pick dates which are good for you leaving plenty of time to recover. In sept I did 5 days b2b staying at B&Bs and carrying everything with me - it was a doddle to organise, a pleasure to run and there were plenty of places to pick up food/water along the away.
SWCP have an excellent website giving details about each 8-15 mile section and have a tool for calculating distances between places. I'll definitely be back to do a different stretch at some point.