Does immigration really make a note in some sort of database as to the reason you are entering, when you entered, when you plan to return to Canada and how long you were gone?
Starting in October they started to use the same entry system for Canadian visitors as for everyone else, so yes they do keep a detailed entry record (for 75 years) and if they questioned you about your reasons for entering and so on they do make notes of it. If you said "hey I'm coming back next week" there is a reasonable chance the CBP officer has put that in there. If they are really suspicious usually they will require you to fill in an I-94 (arrival/departure record) and time limit it to whatever time period you said you wanted to enter for.
However they don't yet keep a record of when you left if they didn't issue you an I-94, unless you have one of the new NEXUS cards and the POE you exit by has the new RFID equipment in place.
This is why I am dubious about the advice immigration consultants give about entering as a visitor then applying for AOS. The situation now is not the same as it was a few months ago. And in a few months time the situation will not be the same as it is now.
Also, if we have been engaged for several months and in my interview I state that on my last visit he just proposed and we decided to marry right away, will they want proof of when the ring was purchased and etc. as well?
It depends on who you get at the interview and how much of a jobsworth they are and it also depends on whether they take issue with the fact that you entered as a visitor (which they may not even be bothered about). Plus now as explained above they will have more information to hand as to what you said at the POE.
I wouldn't worry too much about how far you're planning on driving, I see cars with Florida plates for example at Sweetgrass POE all the time. The only thing that's relevant is whether you can convince them you have non-immigrant intent. Not having a job might be a problem if you then tell them you're visiting your boyfriend.
I did have that situation myself a few months back, the CBP officer was absolutely convinced I was going to visit a girlfriend (even though I wasn't) and I think if I hadn't been able to convince her I had a job in Canada I was coming back to I would have been in trouble.
If you're taking your car in permanently you also need to formally import it in order to be able to swap the title over and get it registered in the US. Obviously if you say you want to import your car to the CBP officer that's going to be a tip off that you plan to stay!
Steve.