(This will make sense only if you've read this post.)
1. It's suspicious that both my syllabic -eds occur in participles that end in -ied. Maybe that was the only spelling available for this sort of participle: I mean, it would look strange to write "unweari'd" or "empti'd." I still think we need "unwearied" with four syllables, but maybe -ied could represent both a monosyllabic and a disyllabic pronunciation. Then we can have disyllabic "emptied" and no need for emendation.
2. Besides, if an interpolator added a syllable to the "emptied" line to fix it, why didn't he do the same thing to the "unwearied" line?
3. And does it really make a difference to the meaning whether we have "this folk" or "its folk" or just "folk"?
1. Plug just "unweari'd" and "empti'd" into Google! You'll find that poets did use such spellings, and the implication is obviously that there'd be an extra syllable if the e were included.
2. Interpolation is a haphazard business. Maybe the metrical "problem" wasn't as noticeable with "unwearied" at the end of its line. And maybe there are variants for this line too: we'd have to check a critical edition.
[Here's my own badly interpolated variant, just for fun: "And, happy melodist, unwearied and." Notice how I "repaired" the meter by repeating a word already in the line, just as in the case of the "this folk, this pious morn" variant.]
3. Oh come on. It's worth establishing what Keats actually wrote, and it's kind of surprising that so famous a poem has an unstable text. The variants would be relevant to someone studying Keats's versification (hmm, do English lit people even care about things like that anymore?).