several questions: refusal @ aiport, proof of funds, tourist visa

I am a Canadian citizen (born and raised) now living in Bolivia, South America. I am engaged to a Bolivian who works and studies here, and we plan to get married in Canada with my family in one year. Then we will both return to continue working and studying here in Bolivia.

I have a few questions:

1.- The Embassy in Lima says there is no specific amount they require for "proof of funds" and its determined case by case. However I wonder if anyone could give me a ballpark figure? a general idea? We will stay with my parents, all costs covered and my parents will say all of this in their letter of invitation.

2.- I was also told that even when they issue the tourist visa here, it must be approved at the port of entry by the agent. That means, it could be refused. Does this happen often? What would be reasons be for refusal at the airport? Is there anything we can do to make sure this doesn't happen?

3.- Is it also helpful to show my work status here in Bolivia as a reason why we WON'T be staying in Canada past our visit? Would this help the situation for my future husband to prove we aren't trying to stay in Canada?

4.- My boyfriend wants his older brother to come along for the trip and wedding for moral support. His brother works but is not married. He is about 40 years old. He can show financial ties to Bolivia and will have the round trip ticket, letter of invitation, etc. the same as us. Could this negatively affect my boyfriend's application? Would they accept one brother but reject the other? Should we have them apply seperately or together?

I know it's a lot but I have so many questions!! Anything people can suggest would be great! Thanks for your help!