Need to agree with the first sentence, disagree with the second. Of the two options, learning to use the DA for the first shot is the preferred option. However, it is quite possible to cock the hammer for gunfighting purposes, although it is more subject to problems.
Concur. Pick one method and then practice the hell out of it. The DA first shot method will be easier and smoother to use (initially), but require more practice to be accurate.
The thumb cock mode (to SA first shot) will be slower and require more practice to deliver a fumble free first shot...but will potentially deliver a more accurate shot with less practice.
As to whether your Ruger is safe to carry hammer down & safety off, I'll admit to little experience with current Ruger semi-autos (and defer to owner opinions).