Yes, if the child is a full time student and you pay more than half of his support. the child can be claimed up to the age of 24.
No, a form 9465 can be filed and this will let you make payments on the amount owed. You will have to pay interest, though. Yes, it is taxable.
Yes, if you have a power of attorney or she is deceased.
Here are some more tax links for you:
Taxing Subjects
or telephone 1-907-344-7046
Uncle Fed's Tax Board
Taxweb for searching
Tax help online
Tax Prophet