| You're right that there is not a lot of authority on this. Pub 17 is probably your best reference. It deals with each of the itemized deductions separately and gives some guidance on separate returns for most if not all of them. (I don't see anything specific on charitable contributions, for example, in either Pub 17 or Pub 526.) Generally, each spouse can deduct only those items that he or she was obligated to pay AND actually paid. For example, real property taxes on separately owned property can be deducted only by the spouse who owns the property, and only if that spouse paid them. It appears that if one spouse owned the property and the other paid the taxes, neither spouse could deduct them on a separate return. Of course the taxes could be deducted on a joint return.
Medical expenses, on the other hand, can be deducted by the spouse who paid them, regardless of which spouse they benefited. There's an example in Pub 17 where one spouse incurred medical expenses before the marriage, and after the marriage the other spouse paid them. The spouse who paid the expenses gets the deduction.
I think the answer to your last question is pretty clearly no. Pub 17 says "...expenses paid out of a joint checking account in which you and your spouse have the same interest are considered to have been paid equally by each of you, unless you can show otherwise." It's not clear to me what they mean by "the same interest," but if each spouse has an equal right to withdraw funds from the account, it seems the deduction would be split 50-50. However, if you could show that one spouse contributed more to the account than the other, you could apportion it as AEM suggests.
Go thru Pub 17, deduction item by item, and I think you'll find some help if not definitive answers for everything. Bear in mind that an IRS publication is just the IRS's administrative interpretation and application of the law. This is an area that doesn't seem to have generated very much litigation, which leads me to suspect that historically the IRS hasn't paid a lot of attention to it. That doesn't mean they may not do so in the future, though.
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