I would treat this as a DNS rather than a DNF, draw a line under it and move on. You were obviously not recovered from your cold and given you were planning to flirt with the cut-offs anyway, the result was inevitable. Most people would have stayed in bed so that says something about your character, definitely not a fraud!
I think what Ben is getting at is you can't fudge your way round a 100 miler. Most people who can run a marathon could probably go on to finish a 50. Just slow down a bit, walk a bit and before you know it you've got 10 to go and can easily walk it in. When you double the distance this attitude isn't going to cut it, walking the last 50 isn't going to get the job done.
That said, it's still just running. It's easy to let yourself feel tired when the end is in sight, so if you've only done 50s and felt terrible towards the end, I would say a large part of that is because you know you're done rather than because you've gone 50 miles - aka it's back on the mental game. Every time I've ran past 50 I've felt fine at 50 because I know the job isn't done. Every time I've stopped at 50 (or 40, 30 etc) I've felt tired because I'd finished the planned distance. It's the same when you first transition to ultras.
The longer the race the longer you've got to listen to your head asking "why are you doing this?" "what's the point?" etc. So long as you're prepared to ignore rather than give in to these thoughts you should do well.
I think a 30 min rest is ok. In my experience a bit of time off your feet like that can really let your legs recover and mentally you get a fresh start when you set off again. For me "beware the chair" is more of a general aid station thing, if you're sitting down every 8-10 miles then the time lost will build up quickly. Knowing you've got a 30 min rest can help speed you through the early aid stations too, and is something to look forward to when fatigue kicks in around 40 miles.
I wouldn't worry too much about the cut-off, they are usually quite generous and will be set so the vast majority of people will get around. 2 hours is quite a bit of time and most of the route is 'road' so probably an even camber and never too steep even if it is covered with snow. Looking at results the winner was round in about 3.15 which suggests it's not especially difficult, so allow maybe an extra hour on the ice sheet and another hour on the path. It also says the route has a net loss of 500m so you won't be fighting gravity too much!
For balance and falling over, shorten your stride right down, 180 steps per min is the sweet spot and this will feel very light on your feet like you're dancing down the road. At a 5 hr marathon pace these will be pretty tiny strides so you'll be able to nip round obstacles and make adjustments very easily. On top of that, practice using your hands for balance, throw your arms out to the side like an animal would with its tail and with a bit of practice you can get out of quite a few scrapes!
You can get the 310xt for a very good price at the moment. It's an old watch, but ticks loads of boxes the fancier modern ones don't, in particular:
Load courses onto it so you know where to go (map!), great for back up on routes you don't know. I used this lots last week and it was mostly great.
~20h battery life
Negatives? It's big and ugly compared to the modern ones (though once on your wrist you won't notice this!) and the software to get info from watch to computer is buggy. It can be slow to pick up satellites too (couple of mins).
I decided it was good value as a stop gap until hopefully a more modern one which ticked more boxes came along (Fenix?) but to be honest as long as it keeps working I doubt I'll ever want to change it.
Depends how much running you've done imo, if marathon distance is still relatively new then recovery between days is going to be slower. Also depends massively on what you want out of it, if you're just there to enjoy then take it easy and enjoy, only you know what that pace means. If you're looking to push yourself then I would make progress on day 1 while still fresh, but be aware of situations which may affect recovery, e.g. take special care on steep hills to preserve quads. Maybe start out at your weekend long run pace and take it from there.
In my experience the first mile or so of days 2, 3, ... will be a little sore but once properly warm you'll be fine.
It probably helps a bit if you know what terrain is coming up, no point pushing it on the hilly bits only to be tired when it flattens out, having said that most of the Ridgeway is very runnable.