Hi
The £18,600 figure is based on annual salary
Category A is based on your income over the last 6 months at it's lowest level. If it's the same each pay day it's easy to work out. If it's different, look at the lowest pay. Take this figure and then multiply it by (if you're paid monthly) 12 and if the figure is £18,600 and your job in the UK (starting within 3 months of your arrival) will pay at least £18,600 then you'll be fine under Category A.
In relation to your second question, it won't make a difference as far as the immigration application goes but it could make a difference in employment law. May be something worth looking into. You just need to ask your employer if your years service in the States will transfer to your job in the UK.