I'm not crazy about singular 'they' and would usually avoid it when conveniently possible, at least in writing. 'People,' for instance, is fine if you want to say 'have their cake and eat it too.'
But yes, the plural '-selves' sounds as if one person has several selves. So yes, it does seem more jarring after 'someone.'
And yes, it does surprise me that people who have no problem with singular 'they' would somehow balk at singular 'themself.' If 'they' is singular, then the reflexive pronoun should also be singular. That doesn't actually seem too hard to me.
I can understand how it might not be as obvious to German speakers, since 'selbst' is invariable and has no plural, and 'sich' can be either singular or plural.
But I would be surprised if there weren't at least some BE speakers who were more sensitive to the issue of pronoun agreement. 3 votes to 1 (isabelll) isn't an overwhelming ratio so far. I would still be interested to know what other regulars such as Mike E., Bion, Marianne, dulcinea, Spike, Kinky, escoville, Anne, etc. might have to say.