The role of "be" in your examples is as an auxiliary verb. There are no real differences in meaning between sentences written with or without an auxiliary verb but they convey different tenses, moods, or voices.
They will marry in April.
"Will" denotes certainty, and the focus of this sentence seems to be on that certainty, less so on the fact that they are marrying.
They are to be married in April.
Phrasing it this way sounds more like it is their intention or plan to marry in the month of April.
Your other examples:
The page was not found
The page was not to be found
Again, these mean the same (nobody found the page) but it gives a different voice to the statement. The first sounds like the person searching was unable to find the page; whereas the second suggests that the page itself was elusive or difficult to find.
I congratulate you.
You are to be congratulated.
The first two are a statement of congratulation from the speaker, but the third suggests that the speaker feels you may deserve congratulations from a wider audience too.