Congratulations on getting your visitor visa. Just make sure you definitely leave the UK before the expiry date on the visa... you'll probably be fine with 178 days, but just in case your flight back is delayed, you'll want to make sure you have enough time to leave before the visitor visa ends.
My question is if the marriage/ settlement visa is appoved and it should be we have all the documents (iidb) required. Will I have to stay out of the UK for 6 mos or can I come back to the UK
right away?
No, you can apply for the fiance visa and enter the UK as soon you like after you get back to the US.
It's only when you come to the UK on a visitor visa that you need to be careful about how long you are out of the country for... because spending too much time in the UK as a visitor can look as if you are trying to live in the UK without the proper visa.
However, a fiance visa gives you permission to live in the UK with the intention of settling permanently (hence the name 'settlement visa'), so it doesn't matter how much time you've spent in or out of the UK before that, because you are moving permanently.
