1. ALL THE DOCUMENTS which you have submitted so far, if you don't have the originals since you have submitted them, please keep copies including the marriage album and additional pics if you have. The more pics the better it will be.
Also keep all communications between the spouses, make copies, print outs, what ever you can.
2. Why this question, is there something to hide? The easiest memory of the factual stuff is the truth where the memory sensors don't need to think twice to speak. Never mess with the truth, if for any reason the documents and the interview point to two different things, then more investigation and time will be required.
3. Yes, the most important thing to realise is that NDVO is a Canadian High Commission office with Canadian officers, they are trained to educated in the country they serve. So for Indians, they know social marriages are authentic and a big thing. Dont shy away from sharing that information even if you have to share pics of all the occasions, days, ceremonies, after marriage events, trips taken, flight stubs, hotel bookings, selfies, duel-fies (husband and wife selfies together), keep the pics KOSHER ;D
If the husband or the wife cant speak fluent English, do take an interpreter/translator with you.
If the husband/wife is not in the country then he/she don't have to present during the interview, its not a must. It will certainly help the case but its not compulsory or mandatory.
Try to submit a list of people who have attended the wedding and also any emails or communications from people who couldn't have and have sent their wishes