Not sure when the cut-off is, mid-september I believe - Edinburgh would get no sympathy if they requested an extension, having turned down opportunities to fulfil fixtures, especially when they have only raced half of their matches (as of today, 10th August).
It only needs another spell of wet weather (not unknown in this country) and they might find themselves in a bit of a pickle.
I wouldn't think you'd need to worry about replacing Risgaer if he's at reserve, that's probably at his level, and every now and then, he could pull out a cracker and put in a 7-ride performance, as Tress did recently.
Why not - he's said that he will be there, and you can't fault his dedication, even if his performance was sorely lacking.
He couldn't do any worse than the weekend's performance - just three more points and the match would have been won. I'm sure Taylor is capable of that, even short of match-practice.