No need for messenger lines. The core is the important part, and the cover will handle the wear.

The toilet antifreeze recommendation is probably because propylene glycol (pink antifreeze) is tough on neoprene and PVC toilet parts. Ethylene glycol is easier on those materials, but it's quite toxic... Not good to put that into the sewers or ground water.

I was reading the latest PS article about propylene glycol antifreeze and they also mention that it stimulates microbial growth. I'm thinking of using compressed air to blow out the lines this year, and suction to empty the fresh water tank, instead of pink.