#1 I'm happier with the expression he's pushing the envelope as far as it goes.
What does 'pushing' mean in the sentence?
"Pushing the envelope" is the phrase. (To add "as far as it goes" is a little tautologous, as this is implied by the phrase) It means trying to extend one's plans or ambitions as much as possible, up to the boundaries of possibility. It normally implies some element of cheek, unusual creativity, or excessive ambition.
#2 It's not illegal to talk to a foreign power about military matters and military technology if that information is a matter of open source.
What does 'matter' mean in the sentence? Does it mean 'small amount'?
No. It can be translated sometimes as "issue", "question" or "thing" but is also used in a number of idiomatic expressions also e.g. it doesn't matter, as a matter of fact. "As a matter of open source" is quite a stylish expression which I find hard to express precisely in any other way.. You could say instead : "if the information is in the public domain."
#3 In July 1924, MI5 obtained evidence which they believe showed that Sempill had clearly crossed the line into illegality.
What does 'crossed' mean in the sentence?
It's used in an ordinary sense - to go over, to pass over, the reach the other side of something e.g. a street
#4 What's the difference between SECRECY and SECRET?
Secrecy is the practice of keeping secrets. In many contexts, a negative connotation is implied e.g. MI5 operates a culture of secrecy. Certain officials have to take an oath of secrecy and here, it would mean something more neutral, like confidentiality.
Happy reading! Michael