P: There is no way to un-mark photos for republishing in Publish Services.

JK, was there a specific publish service that you need this for? I seem to remember that some, if not all of Jeffrey's publish plugins do already have this functionality.

The plugin SDK supports a fairly comprehensive set of criteria for specifying which types of changes should trigger republishing in the first place. I would think a UI for that would be worthwhile as well (or is that already there? - I don't publish. sorry if I'm poking in again...)

Excerpt from SDK API doc:
Possibilities for republish triggers (or exclusion):
rating
label
title
caption
gps
gpsAltitude
creator
creatorJobTitle
creatorAddress
creatorCity
creatorStateProvince
creatorPostalCode
creatorCountry
creatorPhone
creatorEmail
creatorUrl
headline
iptcSubjectCode
descriptionWriter
iptcCategory
iptcOtherCategories
dateCreated
intellectualGenre
scene
location
city
stateProvince
country
isoCountryCode
jobIdentifier
instructions
provider
source
copyright
rightsUsageTerms
copyrightInfoUrl
copyrightStatus
keywords

customMetadata: All plug-in defined custom metadata (defined by any plug-in).
(plug-in ID).*: All custom metadata defined by the plug-in with the specified ID.
(plug-in ID).(field ID): One specific custom metadata field defined by the plug-in with the specified ID.

PS - The default is "everything that's in xmp triggers republishing".

Summary:
========
If not already supported, consider adding to the request: UI to exclude things that should never trigger a republish, and add options for things that will presently never trigger a republish but should.

If already supported, please delete this post and accept my apology...

I've swapped it to a feature request as the current behaviour is working as designed, but I agree it's a really worthy request. It does seem like an obvious omission!

This would probably be better as a Feature Request, rather than a Problem Report. I do agree that an 'Unmark for Republish' option is needed, and I raised such a feature request on the old site. I also included a request for some form of visual indication to be shown whenever any of the user's Publish folders/photosets contain New or Modified items awaiting (re)publish. It's a bit of a pain to remember to keep checking for changes in ALL the folders/photosets in the Publish Services panel.