As I wrote above I have already stayed 6 mo allowed by my entry stamp.
So? That doesn't mean you can't apply for a visitor visa.
The 6 months in 12 months isn't a hard and fast rule, it's only a guideline. It's entirely up to the immigration officer at the airport or the entry clearance officer processing your visa application as to whether they let you in/grant you the visa.
If they believe you are not a genuine visitor and are trying to use the visitor visa to live in the UK, they can refuse the visa/refuse you entry... And one of the things they look at is how long you've spent in the UK in the last 12 months.
They also look at your personal situation, your reason for travel, whether you have a job and a home to go back to in the US etc.
The fact that you have spent 6 months here already doesn't mean you can't try to come back or apply for a visitor visa, it just means that they might decide to
refuse the visa if they think you've been here too long. If the interview will be in the Spring, it will have been a few months since you left the UK, and assuming you would only be coming for a few days and you have proof that you will return to the US, so you may be okay.
However, I would probably try to see if they will do a phone/Skype interview first before deciding on entering as a visitor or applying for a visitor visa. Phone interviews are common these days and I'm not sure that it would be worth all the hassle for you to get a visa and pay for flights just to attend the interview in person when it could easily be done over the phone for free.
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