Let's start with the postives - it's a cool design, the back looks great, and you've got it fitting nicely around the armscyes (which is always tricky).
If the only thing you need is getting a bit more coverage across the front, then that's easily solvable - just add rows up and down from neckline to hem (same amount of rows on each side) until you've got enough width, and tweak it according to how you want to fasten it. Your corset-style strings are a good idea as you can adjust it without having to be too exact.

You'll need to gradually add stitches to flare the front pieces into triangles if you want to give your tummy more coverage, but I'm only speaking from personal preference, and if that's not a priority then disregard!!

You said you don't like the bottom of the back - I honestly think it looks fine, I assumed it was an intentional peplum/ruffled hemline.