Just bear in mind that Tradesmen are always disagreeing with each other, that’s the nature of the self employed. (an, they all want to be the Boss, emoticon)
Re the two port valve, there are plenty of people, who say unvented installation should always have it fitted,
And,
There are those who point out that the purpose of the two port valve, you appear to be referring to, is to prevent the boiler raising the temperature of the cylinder beyond approximately 100 degrees centigrade.
Now dependant on the make and model of the boiler, it may have a device(a fuse) within it that melts at about 95 degrees centigrade and completely shuts down the boiler, therefore some installations don’t need a two port valve to comply with the G3 regs.
Have you taken the trouble to explain your concerns to the original installer and the Gas Safe bloke?
Tim