"Would" is the hypothetical/conditional form of "will". You use "would" in cases where you are talking about possible future events, and "will" when talking about definite future events.
When I get the money, I will go to France next year.
I would go to France next year, if I had the money.
It's a little artificial, but let's take your examples and imagine them as definite future events, rather than hypothetical possibilities:
"They are going to announce their marriage next week." "I will not be surprised when they do."
"Some of the top executives are going to be laid off soon." "I won't be surprised when that happens."
"Have you heard? She's going to leave that job." "I won't be surprised when she does."
We can take any future example where we would use "will" and substitute "would" if we express it as a hypothetical.
"What are you going to have for lunch?" "Well, I won't have the curry. My stomach is really sensitive to spicy food."
"If we go to that new Indian place for lunch, what would you have?" "Well, I wouldn't have the curry ..."
All that being said, "I wouldn't be surprised if ..." is a common idiomatic expression that you can use anytime you feel that some hypothetical or possible future event is not unexpected.
I wouldn't be surprised if it rained today.
I wouldn't be surprised if we closed that big deal by Tuesday.
I wouldn't be surprised if my girlfriend is upset that, once again, I'm late for our date.
It's already eight o'clock. I would not be surprised if Jim texts me in the next ten minutes to tell me he's going to be late. Again.
All in all, I'm not sure these aren't just variations of the "second conditional", since all your examples could be rephrased as if-then statements:
If they were to get married, I wouldn't be surprised.
If some of the top executives were to lose their jobs, I wouldn't be surprised.
If someone told me that she has left that job, I wouldn't be surprised.
