From the Picard end, someone looking up track 1 from this release would get both the track-specific credits and ALL the release credits. However, if they switched to another release, they would only get the track credits if the same recording was used, and they would only get release credits for the new release.
The words track and recording are used somewhat interchangeably, but they’re not the same thing. Here’s a recording: http://musicbrainz.org/recording/8f8730b7-5b12-434a-85a4-0065b2ca4ab3. It’s assigned as a track on all three of the releases in the Fast Emotion release group: http://musicbrainz.org/release-group/c9fd9ad6-2e63-31ff-b630-6a1017cf92e0. Therefore, any changes made to that recording will appear on all three releases (and any other releases the recording is ever used on).
Basically, a track is always a recording, but a recording can be a track on any number of releases or none (a standalone recording). And there’s great proliferation of recordings which could probably be merged, but we just can’t say for sure if they’re exactly the same or not, especially recordings which have been on lots of compilations, etc.
As for the release credits, it’s not an ideal sitation because some credits would tend to belong to all releases in a release group (usually musicians), but some might only belong to one release (such as graphic design or mastering). There’s not a good way for dealing with that yet, so either you have to apply the credits to all releases, or hunt for the one that has the best credits. So if you know performer credits for each track FOR SURE, enter them at recording level because that will be mostly likely to cross over releases. But if you only know them at the release level, you’re stuck entering them on the releases.
(And there are cases in which you might enter the same artist credit twice, such as “drums” and “additional drums”, but it would only be if something was credited that way on the album, or if there was both a recording credit and a release credit for the same role. Probably yes, you can delete the duplicate credit)