"How do you have a single market without common rules and hence political integration?"

Rules and regulations regarding trade can be negotiated just like any other treaty - but they can be held as that, i.e. treaties - there is utterly no need to devolve national power in order to have frictionless trade and even relatively free movement of labour.

North America has something approaching a customs union, and it's very nearly a common market - but there isn't any remote hint of political integration.

Norway and Switzerland are effectively part of the single market without having political integration, though they have less influence, there's no reason a system couldn't be had wherein they did actually participate.

The real sticking point might come down to commercial courts and arbitration, and for that there can be binding mechanisms put in place, without any need for a Court of Justice which effectively has supremacy in all judicial matters in Europe.

Economic impetus is really easy to argue for and it would be fundamentally easier to arrange for something like this than it would be for political integration.

The EEC is really based on this anyhow.

It's the common market and trade that is really the bedrock of the EU, the political stuff is secondary to the point that were there no economic integration, there wouldn't even be any discussion of political integration.