No legitimate historian regards the theory that Henry Tudor murdered them as credible. Pollard does say in his book that Henry Tudor is the only plausible suspect after Richard, but he also spends quite a bit of time dismantling the theory and poking holes in it, and clearly doesn't believe it.
Even Jeremy Potter (chair of the Richard III Society, and one of the leading Ricardians) was on record as saying there was no evidence pointing to Henry's involvement, and pointed out that Henry was such an astute and brutal political operator, if he had killed the boys, he would have produced the bodies and used them to benefit himself (producing the bodies would have benefitted him in two ways, first it proves they're dead which stops any potential challenge to his legitimacy and stops the threat of pretenders to the throne, and second producing the bodies legitimises the rumours that Richard killed them). Henry had nothing to gain by leaving their disappearances a mystery and letting people speculate that they were still alive, whereas Richard needed people to believe they were still alive.
If you look at the contemporary sources accusing Richard of murder, there's simply no evidence to say that Henry or Stafford started those. That's trying to shore up a rumour with more rumours.
His marriage to Elizabeth of York isn't really relevant since he claimed the crown by right of conquest, not via his marriage, so he basically avoided the whole Titulus Regius issue anyway.
It's beyond plausibility that Richard would happily let people believe he was a child murderer for two years just to prevent a hypothetical potential rescue attempt. And what would his motivation be? I mean, in this scenario, is Richard just such a wonderful selfless person that he'd damage his reputation and endanger his safety just to prevent the possibility of danger to the nephews he kidnapped and had declared illegitimate? If he was worried they'd be used to depose him, well, surely if he was that worried then he'd just kill them and remove the possibility altogether?
There are also multiple accounts that give a great deal of detail (which all match each other) as to exactly how Richard had the murder carried out. There's nothing similar for any of the other "suspects"; even people who believe that Henry or Margaret are the killers have never been able to come up with a theory for how, in a practical sense, they would have managed to have someone gain access to the Tower while it was under Richard's control, conduct a double murder, then escape.
The evidence against Richard is far from solid, but he's simply the most logical candidate. They vanished while in his care, he has the most obvious motive, he has the most obvious opportunity, and he had a proven history of killing people and doing anything else necessary to seize the throne. There just isn't any reason to invent elaborate theories with no evidence to create other suspects. Either Richard killed them, or no one did.