I'd have thought they would be able to renew their application without fees near to the time it expires if necessary. It took us a while to sell our house and we had to renew our application. It was quite straightforward (Nationwide).
If they are FTB's I wouldn't have thought they would have a huge problem if the house wasn't ready in time if they have nowhere to vacate otherwise?
Many of the big developers want you to commit to buying months before the house is out of the ground and I agree it's risky. Is there no way they can wait? I assume you must be looking in an area with high demand?
FWIW, I also think HTB is a bit of a risk too. We looked into it and discovered we would be very restricted with what we could do to the house while the government had a stake in it. Think no extensions etc.
Obviously, if it's the only way to get on the ladder, then it's the only way, but looking at properties they can afford without HTB will ultimately get them a bigger, better house faster, with fewer restrictions. Can they look at something less than ideal now and look to sell again in a few years time? We managed to turn a 15k deposit on a £90k house into £65k on a £200k house in about 7 years.
The middle house was a new build from a local developer. It was really too small, but we bought something we knew other people would want. We paid the mortgage and did nothing else and made £35k in just under four years. We wouldn't have bought new build from say Stewart Milne or Persimmon and their developments in our area are now selling at 25% less than the original owners paid. Any negative equity would most probably wipe out their deposit anyway. This won't work for everyone, but perhaps it's a scenario you could present to them as an alternative?