I'd wager that China and Russia have enough intel on other nations much like the USA and UK do on them.

If my theory above was true he's in more pain than he can imagine if Mossad decide to pursue him.

Israel has always left it ambiguous / unconfirmed that it has nuclear weapons - although evidence is there to the contrary. If Trump has given away details of their weapons locations to enemies of both the US and Israel, and other documents about the construction of nuclear weapons (particularly those relating to the USA's programme), then we are in a whole new ball park. As well as compromising the identities of intelligence sources. He is directly responsible for potential murders of agents. 

I mean this is all speculation, although, if true, it has already opened a can of worms. At best, he may have sold secrets to the Saudis or Iran - at worst who knows who he has given information to.

At present he has the DOJ after, the SDNY, and the Manhattan DA and we are now potentially throwing foreign governments in to the mix. There surely cannot be a way out if a foreign power gets involved. 

That's at worst. At best he just allowed every Tom, Dick and Harry to wander around MAL and look through everything he had there. Why did he have articles that even some of the most senior government officials couldn't access?

I don't think you are able to separate fact from speculation on this subject.

The fact remains it now appears? What is the fact in this?

He had documents, classified and not, that he was not legally entitled to possess. Possessing several of those documents, under the Presidential Records Act, is a felony. Trump is not absolved of these crimes because he was president.

His defenders insist he declassified those documents. There is nothing to suggest he did so through an established process, and that's the ONLY way it would have been legitimate. Even so, declassification doesn't address that he had documents he was not legally entitled to possess.

This can't be overstated. A number of those documents could not even be looked at outside of secure facilities and under no circumstances could they be removed. Not even by a president. (If the FBI identifies who removed them, they're almost certainly facing years in prison.)

Trump is in an astonishing degree of legal jeopardy here and no defense offered so far comes close to getting him off the hook.

He is VERY likely to be in prison for this.

More and more news outlets are now reporting that nuclear secrets appear to have been passed / sold to a foreign power.

The schisms this will cause are shocking. It really doesn't matter if it was Israel, France or the UK the fact is he has betrayed classified secrets.

One thing is clear from the act: the United States own any Presidential records and is supposed to retain possession and control of them.

Upon conclusion of a President's term of office, the Archivist is supposed to assume responsibility for custody, control, preservation and access to the records for that President.

So I agree, Andrew, that he is not supposed to have these documents. Nor are they his possessions. This much is very clear.

I honestly don't see where the act covers explicitly what has happened, nor am I aware of any legal precedent on the matter.

This is separate to the issue of confidentiality, which is an area of which I think the details are still alleged or not entirely clear. On the face of it, as I said before, it doesn't look good. But neither, on the face of it, was Hillary's possession of classified documents on a non-secure server. The 'lock her up' argument was unpersuasive there for similar reasons to why I find it unpersuasive (as yet with the currently available information) here.

With the lack of precedent and the limited detail, I would be hesitant to assert a probability of prison time.

Speculation on Mossad assassinating him on the basis of the information available is, frankly, laughable.