Flashbacks are pretty common and certainly don't have to be in any particular order. That being said, I think the issue with your story outline is that you're not really using the flashbacks as flashbacks per se. Instead you've just created a frame for your story and then you're going back to tell it.
Frames can be useful and fun but in my opinion they're problematic when they're just used as you're using them, literally only at the beginning and the ending. Contrast this with the movie The Princess Bride, which used a framing technique of its own (granted, the regular story wasn't a series of flashbacks, but the device is otherwise the same). It worked fabulously well, at least to me, but a huge, huge part of that was that they kept coming back to the frame over and over again. At key points in the story Fred Savage would ask Peter Falk some question, or else Falk would skip over some part that he thought was too "kissy kissy", and so on. It wasn't just an excuse to start with the present day and go back, it told a story of its own.
On the other hand, a frame that exists only at the beginning and the end of a story generally doesn't work all that well. Just as your audience is getting used to the situation you've thrust your characters in, you're telling them to forget about all of that and go back a few minutes. And if your story is long enough and you don't keep referencing the frame, your audience might not even remember it at all, and soon it becomes extra baggage.
If I were in your position I would consider one of two things:
- Tell the story from the second scene and just skip the first one, or
- Come back to the frame every other scene or every third scene to give the audience an update as to what's going on "right now". This can be especially useful if, as in Princess Bride, one character is relating events to another character, but it can also work in a scene where, for instance, people are in a predicament that at first glance seems dire but, once the story is told, turns out to be pretty handle-able (I'm at a loss to come up with examples of this, although I know I've seen this on TV shows a lot).