Are any of the following pairs unlikely to be used? I usually kern them but I don't know if they're likely to be needed.
You need to kern all of them, they are all very ordinary occurrences in Danish.
Norwegian, as well.
Quite rare but not unseen in French, and really common in Latin.
... and Icelandic, Færoese and Old Norse. WÆ is not used in any living Germanic language, (though it was used in Anglo-Saxon and Old Danish), while YÆ is used in one single Norwegian (Bokmål) word, "høyætta" (alternative form "høyættet").