This explanation is from Cambridge dictionary. It shows that "for" is optional in the first example but isn't optional in the second example. Is it always wrong to omit "for" at the beginning of a sentence (sentence 2) and can I use "for" in sentence 3? In Michael Swan's "Practical English Usage" there is a similar example with "all". The first example has "all" too. Swan's example: "I have cleaned all the rooms except (for) the bathroom." It doesn't say that either is correct after a noun, it says after all, any, every, no, everything, anybody, nowhere, nobody, whole, we often leave out "for".
"We often use except and except for as prepositions to mean ‘not including’ or ‘excluding’. They are followed by a noun or noun phrase or a wh-clause. Both except and except for are correct after a noun:
I like all fruit except (for) oranges. (excluding oranges)
Except for Louisa, who’s away in Berlin this weekend, we’ll all be at the party.
She likes going to most sports events, except cricket matches.