Whosever foot fits into the slipper, that woman will be my wife.
Though English is not my native language, due to years of reading, 'whosever' as a possessive came naturally to me in the above example.
Online search shows that the word is very rare and many dictionaries are getting rid of it (Oxford and Collins hasn't, yet). The common alternative given is 'whoever's'. But many users say that it's better to use 'whoever's' as a contraction of 'whoever is/has'.
Whoever's given that job, they better be good at it.
Is there an accepted usage of 'whoevers' (without the apostrophe)? Or, better yet, why not use 'whosever' more often in the right manner?