1) You pay your taxes in bitcoin to whatever government you are a citizen. The bitcoins you use to pay your taxes may be single signature or multisig, it doesn't matter. They will accept any kind of bitcoins whatsoever. In fact, they would prefer you sent them bitcoins that weren't multisig so that they could then "brand" them with their own signature, thereby increasing their influence over the system.
Coins aren't branded with signatures in some permanent way. There is no difference between sending coins by signing with one sig or with two. If you get sent coins that used to be encumbered by A or (B or C or D) or (D and E) it makes no difference, whatever the inputs were have no bearing on what the outputs can be.
In general though if the government passes a law requiring us to wear shoes on our heads it isn't safe to assume people will actually wear shoes on their heads.
You need something more convincing than "the government will tell us to hand our money over". Getting our gold money took generations, and now we have so many more advantages.