1) question banks like medify and medentry are essential imo. free banks are fine, but not the best. ucat courses that teach you the ucat are not the best imo because i think the ucat is an unusual test, in that you get better by doing only. question guidance can help, but only marginally. majority of progress is from practice i think
2) yes you can retake the ucat next year. you can sit the ucat only once per cycle (ie every year). doing it twice is misconduct and you'll get done for it. i think gap year reapplying students have it slightly more difficult since they are expected to use their gap year to do more. graduate entry medicine i heard is way harder
3) depends on the person how long they want to really, but i started doing bits here and there since start of year 12, then really ramped up after mocks finished (so beginning july). 2 months of solid revision.
4) the time matters quite a bit, but it's not everything. the ucat is kind of a really bad iq test (imo), some people no matter how much revision can't go higher beyond a wall; theres a point where more revision can't get you higher scores. most people recommend 6-8 weeks of full focus revision. id disagree with your teacher that you need >=1 year ucat prep, you'll finish yourself at that rate. yes you have enough time to possibly get 3000, and i wish you the best in getting there.
most people get about the same or 100-200 points ish higher than their ucat prep scores. i know a friend who averaged about 2800 ish in their ucat prep, but then come the test and they got 3400. i wouldnt fret over the ucat now tbh, you got loads of time to do other things like get good a level predicteds, get work experience, volunteering (that can increase your chances, like the ucat)