Hi Emmy! First, take a breath, haha. NOT making fun of you. The process is terribly intimidating and complicated when you first start thinking about how to undertake it.
Common-law is no "better" or "worse" than being married. There is exactly one real difference, and that is the requirement to have lived together continuously for at least 12 full months (and you will need to be able to prove that to CIC's satisfaction). You say you spent 4 months living apart. Had you lived together for at least 12 months prior to that separation? If the answer is yes, then you established a common-law relationship. Being apart for a few months after that does not end the common-law relationship unless you want to end it at that point. However, if you had not fulfilled the common-law obligation of 12 full and continuous months of cohabitation, then you must restart the clock from the time you returned in 2012.
As for where you lived together, CIC doesn't care about that, except for the fact that you have to be able to prove your cohabitation. Living with his mother is fine, but as you won't have both of your names on a lease to provide to CIC as proof, you'll need to put together other proofs. Did you both have mail delivered to you at that location? Did you have a joint bank account? Share a cell phone plan? Can his mother write a statement about the two of you cohabitating there (senior members have to help with this one - I think a letter from the landlord/property owner must be a sworn statement? not sure).
Inland isn't a "bad" option, it's just different than ouland. The biggest drawback to inland is that you are not supposed to leave Canada at all for the entire duration of the application process. The biggest advantage to applying inland is that you can submit an application for OWP (open work permit) with your inland application, which will allow you to work after stage one approval (AIP = approval in principle, stage one approval, estimated at 10 months currently), and the submission of which will give you "implied status" to allow you to legally stay in Canada if your current status expires. It's true that you can't appeal an inland denial, but if your proof leaves CIC in doubt of your relationship, they would schedule an interview before denying you, anyhow, in which you and your partner will have a chance to dispel their concerns.
Common-law couples are under the same burden as married couples to prove to CIC in their applications that they are in a genuine and continuing relationship. This means that, yes, you'll need some photos showing the two of you together. Are there other proofs you can gather? Traveled together? Include travel itineraries. You say you've depended on each other financially? Send statements from any joint accounts. Emotional support? Send examples from any chat logs or phone bills that show you communicating regularly when you're apart. You can also gather statements from friends and family if you feel you need more proof. No one can say just how much you need. I stressed out for months, thinking I hadn't sent NEARLY enough after reading about the copious proofs some on this forum have sent. I sent only a 4 page document with 8 photos and information about where/when those photos were taken, along with two travel itineraries. It was enough, in my case.
You're likely quite fine. I'm sure I stressed a lot more than I needed to about everything, haha. Good luck to you and your partner!
