Horizontal wires between posts is how vineyards grow them, in a sunny site, so why should that be a problem? Two parallel sets of wires between two sets of posts is a common technique. Of course, up a south facing wall is best in our climate.

Wine regions are all on well-drained soils, encouraging the grapes to grow deep roots. Research shows that drainage structure affects grape quality more than soil type. But if you are growing it to eat, well the Dutch grow excellent greenhouse grapes on their claggy clay soils. But if you are tempted to make wine from it, you'll probably find it is pretty pale and tasteless grown on such a soil, unless you are very punctilious about restricting the yield, and pruning off leaves to expose the grapes to the sun to ripen.

Dornfelder is grown as an eating grape - it has large berries. It is also grown as a wine grape in Germany, but I've seen it mainly from areas such as Rheinpfalz which have rather warmer summers than England, and it is a pretty light wine even coming from there. I've seen Dornfelder wine for sale from some English vineyards, and their version is practically rose. So presumably it does reliably produce grapes in this country in a suitable site. I've not tasted a wine I liked from this variety, though presumably some people like it.

I have a south facing wall I intend to grow a grape up, once the builder has come back and finished something off that may involve digging where I want to plant it. I intend to grow a Boskoop or Regent, which are two varieties noted as being especially suitable for getting edible red grapes in Britain. Although there are vineyards near me, I don't seem to have the best local microclimate for ripening fruit - on the windswept plain rather than a suntrap in a valley.