Hmmm….I don’t really know but I honestly don’t think it matters, that much? What was most important about Mary signing the Acts was the symbolism of those signatures; not so much whether they were genuine (so long as she wasn’t also publicly speaking against him).

The reason More was executed, despite his former close relationship with Henry VIII, was due mainly to the symbolism of that act of defiance. He couldn’t, as a ruler, demand all his subjects sign but make exceptions for those close to him. This would undermine the Church of England, and his kingship, in the following ways:

A) it would have made Henry look a fool/incompetent leader, one that could not gain loyalty from his subjects

B) it would have given others that may have dissented but signed out of self-protection, and also rebels, a rallying point/figure to center around, and possibly offer support to– it would give them a symbol of rebellion

I don’t think he thought, in all honesty, that it was likely that Mary would become ruler. It was a fail-safe. However, Henry did care about the security of England (I’ll challenge anyone that says he didn’t– I’m so tired of people reducing the succession crisis/Great Matter to Henry’s megalomania, given the precedence of the WOTR and the civil war that erupted after Empress Matilda was left as heir apparent, I cannot even tell you), and he had known the death of male heirs/family close to him, even after they’d survived to teenage years, with Arthur Tudor and Henry Fitzroy. 

So, yes, I would say Henry viewed England falling into the hands of Scottish relatives or other relations as a worse fate– because it would jeopradize the security of England. 

He knew it was possible, and on what he viewed as the slim chance that Edward VI wouldn’t survive, or would survive but wouldn’t have issue by whoever his future consort would be for whatever reason, he knew a secure line of succession was vital. Mary had been his de facto heir for a time, the North had known her and loved her (and did still), and Edward was his only legitimate son (only legitimate child, actually, by the law and in his eyes, I suppose). If he’d had another, of course then the second in line would be another legitimate son, then Mary (most likely, I think)– but he hadn’t. 

Failing Edward, it was Mary, then Elizabeth for the succession because they were the most likely heirs that English subjects would unify under and accept. They were the ones that could, if duty and tragic circumstance called, secure the Tudor line and continue out the legacy with their own issue. The issue of either of Henry’s sisters probably wouldn’t have; there was neither enough recognition of them by the people to warrant such a thing, nor was it as direct a descendancy as Henry’s own issue (and Mary Tudor’s were solely female besides– hence Edward VI choosing Jane Grey).