I got an Unmarried Partner visa about 6 months ago, although we did use a solicitor. Here's my take on your questions:
1. I come from a biased perspective as half of a gay couple. Gays are targeted in the UK for special strict enforcement (they deny it officially, but the record shows that gays are far more harrassed than straights). It is true that if you are refused
any type of settlement visa, it will be very difficult or impossible for you to enter the UK in any other capacity for a couple of years. That's why it's very important to make sure you have the least chance of being refused. However, the unmarried partner visa automatically attracts the right of appeal if you
are refused. Keep that in mind when you go to the Consulate--if you are refused you can appeal it immediately. While it's not an ideal situation, because you will be barred from the UK until your appeal is heard, which could take up to a year, a refusal is NOT the last word. (BTW, if the worst happens and you are refused, get a lawyer immediately unless you absolutely cannot afford it.)
2. If both of you have been living in the US, they won't expect you to have secured UK employment in advance. If you have, so much the better--but it is not a requirement. What you
must show is that you are
employable and will not be a drain on the UK economy (my lawyer's words). This is done by showing your resume (cv). You might also have to answer questions about what sort of work you plan to do in the UK and how you plan to go about getting work. Answer honestly and you'll have no problems.
3. Technically, they mean "enough funds to support both of you without claiming any public welfare benefits, until you can support yourselves." The amount depends on where you plan to live. I don't have any specific amounts, but if you Google for the cost of living in various UK cities, you may get an idea. Also, sites which cater to people who want to study in the UK often give clear information about what Immigration officials expect in terms of financial resources.
They will also be VERY interested in where you are going to live once you get to the UK. If you don't own any property or have a lease, you will need a letter from friends or family stating that they will allow you to live on their premises until you find suitable accommodation of your own. This letter will need to be accompanied with either the original, or some kind of
certified copy of their mortgage documents/lease/title deed, proving that they own or legally occupy the property.
4. I applied in New York, sorry. But in any case I would recommend going in person rather than mailing it in, if you can afford the time and expense.
5. As a gay couple, we wouldn't have dared proceed without these. And if we had sent the application here in the UK, the letters would have been carefully scrutinized. But as it was, the Entry Clearance Officer at the NY Consulate didn't even look at them. Like you, I believe it is better to be overprepared than underprepared, so I would have at least two letters...one from his side of the family, one from yours.
6. That two year requirement is extremely important. You will need five documents from each of the two years on which you are basing your application. You should have at least two different sources for the documents. But, to echo
misch, I had much more than this, including letters from family/friends addressed to us both, and basically every single piece of mail that had ever come to me at this address. I had a little rolling suitcase full of documents.
Be aware, too, that the ECO may demand evidence of personal savings from you. It is not a
requirement to issue the visa--and indeed, I didn't have any evidence of my meager savings with me when I applied--but they may ask you.
7. Hard to say if you're worrying too much. I was in a knot for two years before I finally got this visa. The stress was incredible as there was no way we could have gotten married, so it was the UP visa or nothing. I have relaxed incredibly since I got it.
On Returning Resident: I'm no expert on this, but IIRC, EU citizens are entitled to sponsor their partners in the same way as UK residents. Check this site for more info:
http://www.stonewall-immigration.org.uk . It's directed towards gay people but the conditions are the same for heteros.