There's no stupid questions. I'm in escort in Toronto and I can answer all of these really specifically to the area :)
Donation usually happens before any action, and can be either discretely passed to her or left in some visible location. Yes, rates vary wildly. I'd avoid anyone on the low end of average ($250/hour is pretty standard here).
TER is great. Toronto also has TERB.cc and CAERF.cc, you will find MANY more options on those two. All three boards are good at monitoring and removing fake reviews, and so are their members. (And fake reviews are pretty obvious, IMO - newly created accounts with glowing reviews of only one woman, only one agency, etc).
Stay off Backpage in Toronto for a first-timer. Seriously. TERB and CAERF are free for clients and have advertising there. There's also Eros, Slixa, and Cupid's catalogue (Redditor shout out!). If you MUST BP, yes, you can leave. If she hasn't grotesquely misrepresented herself, a nominal fee for her travel might be appropriate. If it's clearly not her (a lot of Asian agencies come to mind, but there are others), don't even bother doing that. Bad behaviour shouldn't be rewarded and you might consider writing a review for other people about it.
You could say anything from "I'm sorry, you're not what I expected" to "I'm sorry, I have to cancel last minute". If you choose providers that are well-reviewed this won't be an issue, and Toronto's review scene is extremely active.
Most of the lower-mid+ end women in Toronto will NOT charge less for any specific activity. Sure, you might be able to find someone on the street who charges by activity, but do you really want to get involved in that for your first time? Better to go for someone who charges by time and explain what you want.
Garage ...what?! No, that's not going to fly. There's nothing wrong with letting an escort into your house, especially if you've done your research. No one is going to meet you in your garage, that's super-sketchy (not to mention cold and blatantly disrespectful for her). A happy escort is a better experience.
CBJ is a good idea depending on what you're worried about. Some STIs are transmissible via BBBJ but your well-reviewed, well-connected escort is WAY more concerned about her sexual health than the average civilian woman. She has a lot more to lose. BBBJ is pretty much the Toronto standard.
As a total aside, at 18 don't be shocked if someone asks to see your ID. I'd probably ID you and ask you to cover your legal name but not your birthday and photo.
Honestly, it sounds like you should do incalls or an outcall to a hotel room you've rented as you seem uncomfortable letting someone into your home. Maybe you could consider an agency for a first time? No good agency or well-established independent is going to EVER do any of the stuff you're worried about; it's terrible for business and they wouldn't last.
You really should log into at least TERB.cc and take a look around for whatever it is that tickles your fancy. Reviews are going to be your friend in this situation ;)
Some resources:
Have fun! I promise it's nowhere near as scary as it seems :)