You don't want to be going over 40 hours Seren, isn't that the cut off. 70 miles is big, my biggest week is around that but they're few and far between. 40 is my sweet spot, although I have managed a 54 mile training run.
Ah, it all depends. I had ITB issues last year and found swimming aggravated it slightly (especially when pushing off). But if it doesn't hurt then fill your boots. You'll get around, even if injured, with enough spirit.
Defeat is not defeat unless accepted as a reality-in your own mind.
I'd cancel this weeks running if it's still pulling. It's top end speed you'll lose, if anything, and that won't even factor in the race. I don't think there's any real need to taper for these events either so as long as you're rested for three days beforehand you're good to go. I can almost guarantee that by the end of May at least 50% of those at the start line will be carrying a niggle in some shape or another.