Yes, a C2 is difficult to get. Has your husband a clean tax history for the past 3 years?
You should complete all the sections of the form. There are bound to be follow-up questions, so you want to get as much out of the way up front as you can.
The RCT regulations are changing. Previously a sub-contractor could apply for the 35% refund at any time, and if his tax affairs were in order, get it. Now the deducted amount is to be seen as tax paid on account, and set off against income tax etc.
There will in future be a 20% deduction category, for those subs who do not qualify for a C2 cert but have substantially complied with their tax obligations over the previous 3 years.