If you stop paying on the first, the first may begin foreclosure proceedings. The other disadvantage of no longer paying on your first is the damage that it will inflict on your credit score. If your credit is in fair shape, it is the one thing that will help you move on once you are through this financial tough spot. Having said that, if you are going into debt and borrowing money just to keep the payments on the first current, you may want to consider that you're probably dumping good money after bad by doing so. You should consider that even once the 90 days are up, there will probably be another 2-4 weeks (or more) of back and forth with the bank to complete the DILF.
The IRS has become somewhat cooperative in removing liens against homes if they feel that the liens are obstructing a sale of the home that may favor the taxpayer. Whatever the IRS's reasoning was for removing the lien that secures the tax debt, it is the IRS's prerogative to remove the liens. In removing the liens the IRS doesn't forgive the debt, it has only removed the lien against your property for those debts.
Think of the debt itself and the lien they create as two separate things. Just releasing a collateral (the lien) doesn't absolve the debt. That's most likely what has taken place.
Disclaimer: This response shall not be construed as legal advice. Each case is different. Do not act on this information without consulting an attorney in person about your case. This response shall not form an attorney client relationship.
While I do not disagree with anything Mr. Boulgourjian says, my experience is that remaining current on the mortgage does not usually help in getting the lender to agree with the Deed in Lieu. If you have made the decision to walk from the property, I would seriously consider whether the "possibility" that the lender would agree to a Deed in Lieu in another three or four months is worth the money required to make the payments.
Please note that I do not practice in California and, therefore, this response is intended as general information and not specific legal advice.