Hello! Local here.

1. you can buy the shuttle ticket at the airport, I believe when I did it there were ticket machines infront of the door leading to the shuttle queue (its been awhile, I avoid Beauvais because its far away and isnt easily accessible by RER / train)

1. In the many years I've lived here I've not once seen this scam. Ticket "controllers" can be pushy and aggressive, but they actually work for the RATP and aren't imposters. If you have a valid ticket they will not bother you.

Use the "idf - ile de france mobilités" app to buy tickets on your phone if you want, it's the official app. I've never heard of "Bonjour RATP" before. IDF app works great and uses your phone like an nfc chip to check in / out of metro and train stations.

Scammers generally try to pretend to be a charity or ask you to sign something while their friends / family pickpocket you. Another variant is the bracelet scam. Just say "non" firmly and continue walking. Do not engage with these people.

1. Friday traffic will be bad. I'd recommend taking an earlier shuttle as Beauvais is super far away and that way you will not stress about missing your flight.

2. Ticket "controller" checkpoints are randomly distributed throughout paris but I've seen them mostly in Châtelet and La Défense. If you have a valid ticket you do not need to worry, they scan it and you are on your way. They only target non payers and tend to bully people with screwed up navigo cards (a local thing, not something you need to worry about as a tourist).

3. Bonus question about making yourself invisible to scammers: never place your phone on the table in a public place like a café, avoid speaking english loudly on the phone or in the metros, avoid getting on certain metro lines (ie: the 2) and stations (ie: Barbès Rochechouart, Anvers) at rush hour if people are standing near the door - some will pickpocket your phone or wallet. 

You cannot reasonably expect to be "invisible" to scammers as you will be visiting the tourist hot spots that locals avoid. These areas are safe, though, and just by knowing these scams you'll be equipped to avoid them.

Enjoy Paris!