Sorry for the rushed response exams are tomorrow!
Its true that at Liverpool you won't have as much support as if you were at RCSI. However, you will have no trouble arranging electives or preparing for USMLE. RCSI doesn't give you any electives, you have to arrange them yourself so its no different from Liverpool.
Electives are applied for just like applying for university. You will have no trouble getting an elective, the standards are much lower than for admissions so its just a matter of going to the website, submitting an application.
However, the interesting thing is because there will be fewer people, (there will certainly be another Canadian in the school and most people there will be several) you will stick together and help each other more.
USMLE can easily be prepared for on your own. There are countless websites dedicated to IMGs and you can use those to help u find the right tools. Remember, you will meet people all the time from different universities and you will have people to help you on your way.
The option for staying for the 1st 2 years postgraduation is 100% guaranteed. These years are FY1 and FY2. You are paid very well in those years. You work officially 40 hours a week 9-5 for a base salary of around 26k pounds. In reality you will be working about 60 hours a week. If you are willing to work less sociable hours like night shifts your pay goes up dramatically up to around 45k pounds a year. After FY1 or FY2 i can't remember you are officially licensed as a doctor according to the GMC. Then you apply for specialty training spots. There is a preference for EU citizens (if you get married to a Brit/EU person you get EU citizenship) at this stage, but internationals often do get spots. If you get a spot you are allowed to work as a UK doctor forever. You can even come back to Canada to practice after your specialty training (equivalent to residency) especially if its in Family Medicine.
During your final year in med school, FY1 or FY2 years you can apply to the US and Canada (this is what most native brits who want to emigrate to the US do).