'Surely the point is not what trade will maintain but how will they replace THAT WHICH WILL BE LOST"

That's right. Trade with the US is utterly dwarfed at present by trade with the EU. Let's suppose that, by some miracle, the backstop is not threatened and the Good Friday Agreement remains secure (impossible with a no-deal, but one can always imagine). It will take decades for the current trade volume and balance between us and the EU to be replicated by any deal between us and the US, and even that is supposing that we'll get a deal at all. Which we certainly won't if the many Irish-supporting democrats in Congress get even the slightest sniff of a threat to peace in Ireland. Which they will.

By the way, the chlorinated chicken issue is nothing to do with whether we are going to be obliged to consume chlorine. It's about lower welfare and hygiene standards in the US which can only be compensated for by harsh post-slaughter disinfecting treatment. I'm no fan of the standards in the EU but be assured that they are a damn sight better than those in the US.