What should we use for upcoming event Were to be and are to be?

a) Elections that were to be held on Monday have been cancelled.

This is correct. In the past, the elections were to have been held. They no longer are to be held.

(b) Elections that are be be held on Monday have been cancelled.

This makes no sense. If the elections are to be held then they have not been cancelled. You cannot both cancel and hold the elections.

neither as written, use

Elections that were to be held on Monday HAVE been cancelled.

WERE because it now will not happen.

HAVE I cannot explain with a reference, sadly. It just should be so, perhaps there is a single cancellation event and that has caused this verb form to be used.