The first one is OK.
The second does not work. Some variations are:
It should have come out before we bought the tickets ...
It should have been revealed before we bought the tickets ...
The third sentence could be improved, perhaps as:
It has not come out yet that the company stopped selling the product, but the majority [of their customers] are already guessing that.
In response to the question of how would I say these sentences, here are some possibilities.
I only learnt last month that they were going to get married.
Their engagement [to get married] was only revealed last month.
News about their forthcoming marriage [only] came out last month.
For the visa sentence it depends on who should have revealed or found the information.
Before we bought the tickets the travel agent should have explained the revocation of the no visa requirements.
Before we bought the tickets we should have found out about the revocation of the no visa requirements.
The revocation of the no visa requirements should have been explained [to us] before we bought the tickets.
We should have investigated the visa requirements more carefully before we travelled.
When writing I prefer to not use apostrophes for contractions. That may derive from my school days, but I think they can make it harder to understand the written text.
It has not come out yet that the company have stopped selling the product, but the majority of the customers have already guessed that that is the case.
The majority of the customers have already guessed that the company have stopped selling the product even though there is no official statement.