I think June 16th is right for the automatic extension. You can get a further one, but you have to file a form to get it.
What you've written regarding lines 7 and 21 sounds right. You can write in '2555 EZ' on the line next to 21, and then just write the number in parentheses to show that it is negative (this is how they did it on a sample form in the instructions, so I just followed it).
I'm pretty sure you can send it to the embassy London. In fact, when I took mine in person, the woman gave me a sheet about this, and said they were encouraging people to send them to the office instead of coming in person. Unfortunately i don't have that sheet here, but try the FAQ on their website...it's pretty useful.
I just fill out my taxes by hand, as they're relatively straightforward. I haven't been able to use e-file through the IRS, because it won't let you enter 'non-resident alien' for your spouse, and insists upon a SSN or ITIN. I think some people use Turbo Tax and other programs, but I prefer to go with the free method.
ETA: Here is the link to the FAQ
http://www.usembassy.org.uk/irs/irsfaq.htm. You can send them to either Austin or London.