I just to close out this topic, my fiancee was granted a visa on Feb 4th. For he third application, I spelled out very clearly why she would not/could not stay beyond her visa time (1-that she needed to be in Moscow for her US visa interview, 2-that I would be risking punishment if I allowed her to stay, and 3-that if we intended to violate the visa we would've done so already when she had a valid visa when I arrived in the UK - it expired in Nov.) plus I got letters of support from my US Commanding Officer and from the RAF Officer Commanding of the RAF Station where I work. I don't know what was the tipping factor, but she was called in for an interview and granted a visa that day. Based upon the questions she was asked and the UKBO's reactions, she felt he was inclined to grant the visa before the interview and was just verifying his decision. She will be here in a couple of weeks and spend a month here. As if this process hasn't been difficult enough, now she has an application in with the Irish embassy to get a visa so we can go to Dublin while she is here.
Thank you all for the comments. It has been very informative. If things go well with the US interview in May/June, she will quit her job and return to spend the summer here I return to the US with her. My big lesson from this - don't assume a previous granting of a visa will translate into an easier time getting another visa. Treat each application as if it is for the first time and clearly provide all of the information the border official is looking for (why is he/she coming?, can he/she afford it?, and what is the compelling reason for him/her to return home?). I hope this helps someone else.
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