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First of all - regarding of extension - you are already late for filing your 2014 tax return...The IRS rules - if either spouse disagree to file a joint tax return - another spouse woudl not have any choice - but woudl be only able to file a separate tax return using MFS (married filing separate) or HOH (head of household) filing status depending on circumstances.
You may change your choice and amend the tax return to MFJ (married filing jointly) within allowed statute of limitation period (normally three years)
If the settlement directed differently - and your spouse disagrees - that woudl be a violation of the agreement - and you may sue her for damages - but that is a civil matter between you and your spouse.
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