Dear, Bella!
Nobody here can say how long will the marriage lasts (either yours or theirs) and what is the best answer when it doesn’t happen as we hope….
In your situation, I think, is better to get married to your children’s father and get GC this way, since you already know it would be faster.
Once you get it – you’ll have less worries and staff to think about, with the new baby on the way and everything.
And even if you do end up divorcing very soon, you have a very strong case already (children and many years together), I don’t think you will have a hard time removing your conditions on your own. But, please – think positive. Hopefully things between you two get better, you are a family with kids and that’s what important, don’t let this GC business mess up your life, OK?
Good luck!