Hello again Jenmaries,
1) You can write in the top border of the Form 1040 "Taxpayer Resident Abroad", and then mail the return. If you want to play it safely, which isn't always a bad thing to do, the statement would normally be in the form of a cover sheet. Write your spouses name in the upper left hand, and their Social Security number in the upper right hand, similar to any IRS form. Below that, title it "Automatic 2 month extension..... (wording of your choice)". Below that, your statement, "I was a permanent resident of the United Kingdom from......and outside of the US on April 15, 2010.....(again, your choice of words). For this year and in following years, separately on the cover sheet, include a reference to the unofficial exchange rates you used to calculate the taxes, and the source (IRS, US Embassy website advisory, etc.)
2) Yes, (I'm assuming you're referring to Schedule B.)
3) Technically, you could argue yes (for Sch. B). But I'll add a caveat here. In my amateurs opinion it may be just as expedient, since its March, to calculate the interest from the account as if your spouse owned half of it for the entire year, provided it has no/little impact on their final tax position (tax due).
4) Don't mess about here. In my totally naive brain, money laundering can occur in ways I can't even imagine. The FBAR requests the highest amount in the account during the tax (calendar) year. Put the highest amount the account had in it during the year (pre-March?), regardless of when your spouse was the owner and what the balance was at the time of her involvement. It's safest (in my opinion).