Copper is noble/ cathodic with repect to steel which is more noble than zinc. So a reaction would cause corrosion of the zinc and then the steel. You shouldn't now use copper upstream of galvanized because copper particles could cause corrosion.
Di-electric unions, they call them. Our earth bonding would bridge the insulator and make them useless. They're pretty useless anyway.
They were using propylene glycol I think,which is a component in some anti-freeze mixtures and is also used as a sweetener; that's why they used it. It is non toxic, unlike the more common ethylene glycol, which is very toxic. So you use propylene glycol in domestic heating systems (only if you HAVE to use anti-freeze). And in your 4x4 in the arctic 'cos animals will drink any liquid that hasn't frozen. I learned that while racing sled dogs ;-).
It's the corrosion inhibitor additives that prevent the galvanic corrosion. The glycols don't,they breakdown into some acidic compounds and so cause the galvanic corrosion when the solution's reserve alkalinity is exhausted. So you'd only use anti-freeze in a heating system if it is essential.