But I don't think there is any suggestion MG had a second set of keys, is there? AS's book doesn't suggest that, and DV's goes a step further, making it clear DV doesn't think MG went into the house at all.
A lack of forensic evidence doesn't absolutely guarantee Suzy never went into the house. If all she did was open the front door, walk around a bit, and then come back out, trace evidence could be minimal or non-existent; to quote an old saying, absence of evidence is not evidence of absence. On tv and in movies every person leaves behind copious trace evidence everywhere they go. But in real life it's not always that simple.
MG supposedly got his initial information about the man and woman at Shorrolds, which he reported to the police at 6:45PM, *from* HR. How could HR have been talking about having seen MG and SF when he was actually telling the story to MG and SF? It is possible that other witnesses saw MG and SF, but HR can't have done.
HR reported seeing a man and a woman at 37SR at around 12:45-1:00PM. He told this to MG before anyone knew Suzy was missing and before the police had even been called. He appears to have had no reason--or any knowledge of the situation that would have allowed him--to make it up.
Having now read AS's book, I think HR has been treated somewhat unfairly. The first version of his story, the one he told MG, supposedly included the woman being bundled into a car, which was later found to be at best an exaggeration and at worst an outright lie. But it's not at all clear what HR actually said to MG, or even if it was HR or MG who exaggerated the story to begin with.
HR apparently told a similar but less embellished story to the police; again, who originally embellished the story, him or MG? If it was MG, had someone (DL, whom MG had already spoken to before calling the police?) encouraged him to embellish the story hoping the police would take the situation more seriously?
You also say HR positively identified the Belgian Mr. Kiper as the man he saw in Shorrolds. But according to AS's book, all he could ever say was that the man he saw *could* have been Kiper, not that it definitely was. HR had apparently been given an expenses-paid trip to Belgium by a newspaper to look at Kiper, so it's hard to know if his answer was even genuine, or just an attempt to appease his benefactors.