First, in Texas, there is no such thing as legally separated. With the exception of DOMA (which presumably doesn't apply to you), the IRS uses state definitions. So you are not legally separated for any purpose, tax or otherwise.
You are married and have to file as married. If you mark that you are single, you will be filing a false tax return. I don't know the penalties for that, but messing around with the IRS is a really, really bad idea.
You cannot file single until after your divorce is finalized. The relevant event is the granting of the divorce, not the filing of the petition for divorce or living in separate household. However, your wife is not necessarily entitled to claim all of the tax incentives. You should consult with an accountant regarding this matter. Regarding access to your wife's income, you can get this through discovery in your divorce proceeding.
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