I haven't read all pages of this thread so if my suggestion has been mentioned, please forgive me.
I like to make my cast on, for top down socks, with a needle that is 1-2 sizes larger than the needle I use for the socks.
Example: With a size 4 options interchangeble needle (tip only - no cable attached) I cast on my 64 stitches. Be careful not to let them slip off either end of the one needle. I then take the size 2 needles with a long cable attached (because I work the magic loop method), place the pointed tip of one needle into the opening of the #4 and then slowly slide all of the cast on stitches onto this needle. I am then ready to procede with the making of my socks.
I don't like using dental floss as a lifeline. I use cotton crochet thread instead. It works well with the interchangeable needles. My problem is that I don't always remember to pull it out of the hole after I reach the end of the row I am working on. Otherwise, it does a great job. I also use as many as three lifelines. I leave 1-2 in place at all times.