Firstly, you could use the single basket with the double spout, it just means your cup would need to be wide enough to capture both streams of coffee. However, most people here dont bother with the single basket as getting a good coffee out of it is a bit more fiddly.
What I often do if I am only making coffee for myself is either have a double (why not!) or I taste the second shot without milk as it helps you to tell what (if anything) went wrong. The milk tends to cover over any mistakes in your extraction.
If you are finding the group handle difficult to lock in to place, it means you are over filling. the newer 6910s have a lower showerhead, which means you need to play around with the fill level until you get it just right. When you can lock the group handle in without any resistance, remove it before pulling the shot and see a very slight imprint of the showerhead screw on the dry puck, then you know youve got the level *just* right. You could even back off a 10th of a mm and you should be right.