1. MedEntry is just a preparation course that gives you a whole bunch of questions that are meant to be similar to the ones in the actual exam. They have solutions as well. It seems to be the preparation course that most people will talk about.
Just be aware that the questions on MedEntry do not always reflect the style of questions seen in the UMAT, especially in S2. I found it most useful for S3.
2. You can begin preparation any time you want. I'd say the June-July period is the most important though.
3. UMAT is a very unpredictable exam. The amount of study you do might not always pay off - I've seen people dedicate so much time for the UMAT who unfortunately didn't make the cut-offs, and I've seen people who did little to no study for it and still got above 95th percentile. It's mainly meant to test your natural aptitude and EQ. Luck plays a little bit of a factor too.
As said above, there are always other pathways into Medicine. The GAMSAT is held for graduate medicine, and I've heard that it is more "preparation/study friendly" than the UMAT. Either way you go, good luck.