Do native speakers ever (erroneously) pronounce past participles in -thed, such as bathed and clothed, as */-ðɪd/ rather than /-ðd/? If so, how prevalent is this?
In Modern English, a big No. In Old English and Middle English, Yes!
The past/past participles of regular verbs were all pronounced /-ɪd/ (or maybe /-ɨd/) until Modern English. As far as I know, Shakespeare has used both forms (/-ɪd/ and /-d/), but the vowel was then lost and the final /d/ merged into the preceding consonant. That's why we say /beɪðd/ not */beɪðɪd/.