Can I get half of my wife's 401k when we get divorced?

It is possible that you are due many of those things. Your best bet would be to contact a divorce attorney.

Get an attorney, your positions appear reasonable.

Once you sign you are bound to what you signed since you did so knowingly. Your failure to read what you signed is not her fault. In any case, you are not necessarily entitled to half, you are entitled to an equitable (fair) share. What that means involves a variety of factors including each of your earning trajectories. So if you earn more than she does (which is likely given how much you've accumulated in the 401(k) is a short time which she's taken many years to accumulate what's in hers) a court would likely give her more than half of the assets and give you more than half of the debts.

Yes, but remember she gets half of yours too. Of course, it is only of the amount accrued during your marriage, not the whole amount unless entirely accrued during the marriage.

I there is no mention of the 401-K in your agreement, you may be able to seek your fair share before the final divorce. In other words, if you didn't already sign an Agreement which stated that you waived your right to claim your marital share of your wife's retirement funds. Otherwise, you will just have to challenge the Agreement as you didn't have legal counsel to advise you, and you didn't understand what you were signing. The latter course of action is more of an uphill battle.