The most efficient approach would be to simply convert over to using linked images, which won’t be much additional trouble, since you’re going to have to go through every image anyway one by one, to change the DPI. The process is described in this post in more detail.
As for a more general description of the kinds of image handling options Scrivener makes available, there is this post.
Juggling multiple projects and keeping revisions and typo fixes up to date in both sounds like a real headache and source for unintentional errors, for something that has a good, designed solution.