Is it possible to get a data dump for Guitars like all the other sites?
As far as I know, this isn't possible.
If not, what options do I have in identifying which are the merged posts?
It's not likely you can find all merged posts in a simple way, but here's an idea to get started. Question #1 on Music Stack Exchange, Criteria for selecting a voice teacher was posted on April 26th, 2011. If you search for questions posted before that date with the query
created:..2011-04-25 is:q, you get 374 results; questions which must have been posted on another site. You'll note that the timeline and the revision history of these questions do not mention any migration events, which is another sign of the sites having been merged.
Of course, if the sites coexisted for a while, it'll be much harder to weed the newest questions out. Something that might help is that the merge does not create new tags on the target site, and merge-migrated questions might end up getting [untagged]: https://music.stackexchange.com/revisions/1061/1.