An ancestry visa is not available to her because she is not a commonwealth citizen I'm afraid.
She can't travel here on a visitor's visa (ie. 6 months granted to US citizens upon port of entry) and then apply whilst she is here because it is a term of her visa that she will return home and not attempt to secure further leave to remain in the UK whilst visiting.
Realistically, if your decree absolute came in three months time (Mid-May), you married immediately afterwards (say end of May) and made an application immediately for a spouses visa then taking account of the 8 weeks it takes the UKBA to reach a decision you could conceivably have her spouses visa by beginning of August. The difficult with this way forward is that the UKBA some frequently times take longer than the 6 weeks to approve. If they do not approve then you would have to appeal which would take a further 2-3 months. This would put your fiance's job starting date in jeopardy.
The safest way forward to get her here and able to work in the start of August would be for her to make an application for a Tier 2 visa based on her job offer/certificate of sponsor ship.
Check that she meets the points eligibility criteria on the following page:-
If she has a certificate of sponsorship, a BA and access to funds then she will have no problem meeting the criteria.
Once you have the job offer and certificate of sponsorship she can make the application and the visa will usually be granted to allow her to come here for two months before the job actually starts. If it starts on 1st August, I would expect the visa to be granted from 1st June.
She will need to submit form VAF9 to the British Embassy nearest to her in the US. IT would obviously be best for a solicitor to prepare the application and supporting documents to give her the best possible chance of a swift approval. It should cost around £800-1000+vat for this.
You can find UK solicitors near you using the following site:-
If you wish to see her in the meantime then she could come now for a short visit and bring her supporting documents to take to a UK solicitor to start preparing the application. If not, then she can send them to you/the solicitor.
She will need to submit her passport when making the application so will not be able to travel until a determination has been reached by the UKBA.
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