The apostrophe has largely disappeared in the names of businesses...hence, we now have Barclays Bank rather than Barclay's Bank. The presence of the final 's' does not really make it a plural word, but it may be seen as such. So the use of 'is' or 'are' is optional. We might say, "Barclays is my favourite bank" or "Dovercourts are always reliable in my experience."
"Dovercourts are the one" or "Dovercourts is the one" are both perfectly fine as an advert in my view. "Dovercourts are the ones" doesn't work for me, though.
This is exactly what I was saying in the other thread you mention, so you will probably find the same people appearing here to contradict me!