2) your attorney is right that sometimes it can be better to file early in the US as well. That will be the case when there is a danger that anybody in the US will file a patent application before you would od (i.e. during the priority year). In such a case the earlier filed US application can be held as prior art to your later filed US application, even if you would be able to claim priority. Or, in case that your US application goes beyong your first UK application you will not be able to claim priority and thus the earlier filed US application will win anyway.
However, if such would be the case (i.e. the danger of a third party filing early in the US) your attorney should have insisted in your filing US as well, since the mere act of filing a (provisional) US application is rather cheap.
3) you may change patent attorney. However, if you are buidling a portfolio of applications (i.e. in different countries or more than one invention) it would be advantageous to let one attorney coordinate all the filings and prosecution.