I don't think the schwa is "added" through epenthesis. The original genitive and plural endings were actually /əz/ in Old and Middle English. It's just that in Late Middle English and Early Modern English, inflectional endings disappeared or were reduced, so the original /əz/ turned into a simple "s" that was assimilated with the preceding consonant or vowel in terms of voicing. The only place where it persisted was after a sibilant consonant (one of the hissy consonants /s, z, ʃʒ, tʃ, dʒ/), because "s" is itself sibilant. The same principle applies to "ed", which used to be /əd/, but was reduced to an assimilatory "d" that only persisted after an alveolar stop (/t, d/) because "d" itself is a stop. The only ending that was gone for good, without any trace whatsoever, was the infinitive verb ending /ən/, meaning that we don't say "worken" any more, but simply "work".
I don't think the schwa is "added" through epenthesis. The original genitive and plural endings were actually /əz/ in Old and Middle English. It's just that in Late Middle English and Early Modern English, inflectional endings disappeared or were reduced, so the original /əz/ turned into a simple "s" that was assimilated with the preceding consonant or vowel in terms of voicing. The only place where it persisted was after a sibilant consonant (one of the hissy consonants /s, z, ʃʒ, tʃ, dʒ/), because "s" is itself sibilant. The same principle applies to "ed", which used to be /əd/, but was reduced to an assimilatory "d" that only persisted after an alveolar stop (/t, d/) because "d" itself is a stop. The only ending that was gone for good, without any trace whatsoever, was the infinitive verb ending /ən/, meaning that we don't say "worken" any more, but simply "work".
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