1 If the home you are buying directly replaces your main residence, you will not be liable for the 3% surcharge, even if you own an additional home/s at the same time.As this property is not your main residence (your main residence is in the US and this property was let) you have to pay the 3%.
2 You have to use the value back in April 2015. I am not allowed to advise or recommend on a specific company or person unfortunately.
No you will not. If you have only the one property then you are not charged the additional tax.
Anyone who is buying an additional residential property has to pay the tax. If you sell the only property you own then no, it does not apply.
When you sell the one you own now you have no other property. Purchasing another then would not trigger the tax because you would not have any other property.
Setting up a company would most likely just complicate your situation. The letting would still be reported and you would have additional filing in the US for having a foreign company.
No it would not.
Let me see if I can say it in a different way for you. If you own only one property on the day you purchase the buy to let then you are not subject to the 3% additional on stamp duty, no matter what you use the property for.