What is the level of formality in the expression “if and when” while referring to a possible happening? Googling “if and when X happen” gives me all sorts of entries, some formal some clearly informal.
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If and when carries no direct formality implication as your search shows.
Note 1: Unless you associate the complexity and precision with formalism. As in if you assume that a formal audience will more likely understand (and also correctly use) the expression in all of its subtleties.
Note 2: The phrase was actually heavily criticized by Fowler (see here) and perhaps more appropriately treated in Merriam-Webster. This complexity is something that you might not want to deal with in informal text or talk.