I once had a baby rattle (when I was a baby). Why is it reasonable to presume I still have it in my possession? I can't prove that I don't since you can't prove a negative.
You wouldn't have to prove you don't have it - you just have to show enough evidence to show that the question is in issue. If in the circumstances it is reasonably likely that it isn't in your possession - such as where you have not had it in your possession for years - the question is in issue, and the prosecution will have to prove it beyond reasonable doubt.
As for 3b, he told them his best recollection of the password and it didn't unlock the drive. So there we go, where is the proff that he does correctly remember the key but chose not to tell them?
I don't think there's much point in speculating as to whether he did it. The person who saw all the evidence and who was able to listen to Wilson and assess his credibility was the judge - without any evidence I don't see how we can really question his judgement.
Frankly I think giving 50 incorrect passwords is more likely to be a sign that you were being obstructive than that you were genuinely trying to remember but couldn't, but again - I don't know, because the journalists didn't report any details.
There may be indications and reasons to suspect, but the standard for jailing someone is proof. Where memory is involved, there can never be proof. At least not with today's technology.
The standard for jailing someone is proof beyond reasonable doubt - not absolute proof. I presume that there was sufficient evidence for the judge to conclude, beyond reasonable doubt, that Wilson was lying. If not, I hope he will appeal and be vindicated - but we haven't seen any of the evidence so we don't know.
I would say that courts deal with lots of people who say they "can't remember" or "don't know" something, and have to decide whether they are telling the truth or not - whether that's people who can't remember where they were when a crime took place, or who don't know where some money went, or a million other possibilities. It's a difficult question, but it's an inevitable one for a criminal court to grapple with and they have plenty of experience doing so.