Hello Oscar, welcome to the forum
1. Her family doesn't need to be at the wedding, but there better be a really good reason for it. For example she was having surgery, or can't travel because of medical condition, etc. If they're not there, you will have to explain to them and convince them that they couldn't assist to this very important and life changing decision of their daughter for a very good reason.
2. It doesn't matter when you two met each other, but if it's too soon (if you've only been together for a month) it looks a little suspicious to their eyes so you will have to explain that too.
My two answers are "probably", from what I've read in this and other forums. I wasn't asked for an interview so you could still pull it off without it if you find a way to provide evidence that your relationship is genuine and continuing (<-- this is very important)
To get married in Mexico
First, be prepared to spend a lot, and I mean, A LOT of running around. Every civil registry office has similar requirements, although they may differ a bit so your second step is to go and request from the office you're planning on getting married, a little sheet with information on what you need to bring to them. Be sure to mention she's canadian so they give you the additional documents you need to gather.
Thirdly, the general documents that are very important for her to bring:
1. Legalized and translated (by a certified translator) birth certificate.
The legalization of a document is done by the mexican embassy in Canada, and the translation is on you, I would suggest you do the translation in Mexico because if you don't, you might need to legalize the translation as well (we weren't asked for the legalization of the translation).
Note: Canada doesn't do Apostille because they didn't sign the Hague convention.
2. 2600 MXN for the INM permit to marry a foreigner, and about 30 days to wait for its elaboration. A lot of patience with the INM will be necessary too since at least to us, they had us coming back and forth in the Chiapas jungle for 5 days (we even had a small car crash one of those times).
The letter is from INM, not from SEGOB btw.
3. about 100-2500 MXN to schedule a minister to go marry you at your preferred location. It is actually worth it to make a very pretty wedding (and hence spend the 2500 on the minister), because it shows how important it is for you two to get your lives together, and CIC will take that as a "hey, they care" instead of: "hey, they might be abusing the system". I've read of a lot of people who've been denied their applications because they got married at the Civil Registry offices.
4. Medical tests. It needs to be done by a mexican lab, and you need to tell them it's "marriage medical tests". This document expires in 15 or 30 days depending on the lab, so don't do them until your wedding date is near.
5. The migratory form she gets from border services at the airport when she arrives.
6. Photocopies of her passport, ALL of the pages.
7. 4 witnesses, 2 assigned for you, and 2 assigned for her. They can't be the parents of any of you because the parents need to sign separately (if they're absent they just write "absent"). Other family members are alright.
The witnesses' ID's usually need to go next to yours on a special arrangement the civil registry offers operates under. Ask about that at the office too.
My following advice is to just take the time to read the forms. We read every single long a$$ document we could find on their page, and gave them all the information they required on the proper channels they requested, and as a result our application went up smoothly and I'm already here.
Good luck with your application and don't hesitate to ask more questions!
I'm currently packing and doing some inventory to fill out the B4/B4A
