"... Name one obstacle ..."Simple enough ... VOTERS!!! With enough votes coming from the grassroots movement to pass the FairTax into law there will certainly be enough to keep the income tax from returning by a rhetorical snap of the fingers as you suggest.
Not only is the tax code removed by the FairTax, but the IRS is defunded and disbanded and the income tax records will have been destroyed. Attempting, then, to restart that from scratch against what is sure to be massive taxpayer opposition since voters will greatly like the FairTax (in comparison to the prior income tax) and realize that it benefits both them AND their country it is HUGELY unlikely that they would tell their Congressman "... sure, I like having the higher effective tax rate under the income tax so I can pay more in taxes than I've been doing under the FairTax while saving and investing tax-free ...".
SURE!!!
If they made it only an income surtax on the "evil" rich, it would be a piece of cake if the situation required it-- like everyone realizing the FairTax caused all prices to go WAY up, and the economy to tank...
Probably just a flat 25% on everything above $50,000 or $75,000 to fix the emergency. What other choice would they have?