You may want to check out
British Expats Forum, hopefully they can answer your questions. UKY is more for US to UK moves but we do have a Repatriation Section you may want to check out.
In my experience we filed our I-130 on 7 June 2010 and we received a request for further information in Late August and returned that the day we received it and I received the approval notice in mid September but my husband did not receive his notice from the Immigrant Visa Unit until the first part of October. We currently do not have our documentation together at this point so we have not notified the Embassy that we're ready but we've scheduled a medical and are just waiting for some paperwork to arrive from the US. So the 9 months estimate is pretty accurate (so far at least for us). Once we have our paperwork & notify the Embassy it takes about 4-6 weeks to get an interview appointment and then they make there decision that day and we should (fingerscrossed) get our visa a week later. We were hoping to be back in the US by Christmas, but it's looking more like late January or early February at this point.
I believe that to be able to file the I-130 with the US Embassy in London your wife must be able to show that she is normally resident of the UK (being here at least 6 months but some have even been told that they need to hold ILR before this is possible it just varies depending on each case). The Embassy is really not keen on those that come to the UK on a spouse visa and then file the DCF request because it's really a short cut so they like to see normal residence established and may question you regarding this.
That being said she can file the petition with the Embassy first and then return to the US to set up home (notifying the Embassy of the change of address for herself once she relocates) and I do believe that you can enter the US as a visitor for up to 90 days but you would have to return to the UK before that visitor period expired. But if those 90 days pass without approval you would have to return to the UK to wait out the petition processing time and in case there are other documents that are needed they will need to be addressed as well.
I'm not sure about the K-3 visa you would have to explore the other forum for those answers.