I have always been disappointed in the idle of my M103 engine. The only thing that I ever did that seemed to improve it, was go to a little tighter plug gap and took out the platinum plugs.
I went to a copper core, standard Bosch plug and gapped them at .034. This helped noticably. The platinums have a tiny center electrode which makes a thin spark.
My Lambda sensor test has always proven okay, right at 50%. I'm sorry I can't help with your other questions, but I thought my spark plug experience might be useful for you.
Have a great day,