If you don't begin with the MC, you will need to begin with someone important and have a good reason. As with any rules, it can be broken but you have to understand that until told otherwise, the first character introduced is the MC in the reader's eyes.

How much of the story should you reveal? Well, as the story isn't over yet, you should reveal what is going on at the beginning. You can't reveal the middle or the end because they haven't happened yet. You should not withhold information that the POV character knows that is true at the beginning -- although it may take some time to get around to putting it on paper.

1.
In my novel, I begin with a prologue that shows the MC in a scene that is super-duper important to her life and to the entire plot of the novel. The scene must be known and understood by the reader before getting to the rest of the novel. It is years and years before the rest of the book, hence putting it in a prologue.

The problem is that the scene is hugely emotional and dramatic for the MC - too much so to be the very first thing that you encounter in her POV. You wouldn't know her or care about her enough to care or understand why this scene is important to her. Now, since prologues are "allowed" to use a different POV, voice, etc. than the text proper, I decided to show this scene from the POV of the other character present. Other than this scene, he is never a POV character again. But he is an important person to the MC, and present throughout the novel.

By making this choice, I discovered a delightful, unexpected side-effect. Now, through the entire novel, we know what that scene meant to this other character who we see but whose head we are not in, and it is vastly different than what we would think of him if we hadn't seen it from his POV. So much so, that it squarely dictated to me what the last scene of the entire novel needed to be: one small scene between him and the MC, from his POV, resolving his feelings from that prologue.

2.
I discovered something else the other day. I have spent 18 months and 16 drafts trying to find the opening to my novel (chapter 1, not the prologue). I finally found it last month, and began writing. Part of the problem was finding the right event and moment in the timeline of the story to start with (the moment when the MC first gets involved). I found it, and knew instinctively that I was off to the races. However, about 10 pages in I started introducing information that was central to the entire plot of the novel, but hadn't mentioned yet. I needed it to already have been introduced by the time I got 10 pages in. I realized that this information needed to be up front - in the very first scene, and in the first 13 if possible. BUT that didn't mean I hadn't started in the right place. I had. But I hadn't included the right information. (Essentially, I had the right character at the right time in the right place, but I had written her thinking about the wrong things.)

So from this I learned the important lesson that an opening must include 2 important things: it must start in the right place in the story (including the "correct" POV), and it must introduce during that scene the central meat of the story, whatever that means. Those are not necessarily the same thing.

But the biggest thing I learned from this (though it is slightly off-topic) is that there are things you have to solve in the planning stage (where to start the story) and things you won't learn until after you start writing.