Transfer (all) Keywords from one Photos Library to another

To move them:

Select any pic. Assign all the Keywords to it. Export it to the desktop. Switch libraries. Import the shot into the second library.

Assuming you want to use Bruecke as your keyword.

Make a smart album for Keyword is Brücke. Select all the images in it. (Command-a). The Get Info and simply type over the existing keyword with the preferred version.

Are you able to create a PDF file for that book?  If so there's a couple of 3rd party book providers one of which used to do printing for Apple, Mimeo and also PrestoPhotos, that will take PDF files created by iPhoto or Photos and print and bind books. Use the Mimeo link for direct access to the Apple Photo Project Ordering Service web page. 

Create the pdf file according to this Apple document: Preview a print project in Photos or iPhoto to avoid issues - Apple Support.

In Photos 4 projects are copied over to other libraries only as albums with the photos for those projects in the album. There is no know why to transfer a project intact from one library to another. That's why I use separate library for each book and/or calendar I create. If you need a pareticuar project on another Mac just copy the appropriate project library to it. They end up being relatively small. I assemble the photos I want to use in an album in my primary library and copy it to the other library with PowerPhotos.

Second question: I have 2 keywords (like Bruecke and Brücke, all of them used in some images.

I cannot find a simple way to merge them in one step).

After you replaced the wrong keyword "Brücke" for all photos by "Bruecke" using the smart album, as TD described, you may want to delete the wrong version from the Keyword Manager.

Open the Keyword Manger panel with ⌘K and then click "Edit Keywords" at the bottom of the panel. Select the wrong version and click the "-" button to delete it from the list.

But if you assigned the keywords in Aperture and used Aperture's batch change command "Metadata > Batch Change > Apply to original files" to write the keywords into the original versions, they are burned in forever. Even if you replace the keywords in Photos by he correct versions, the changes will not be applied to the original files. Whenever your library will be repaired or upgraded, or the originals will sync back with iCloud from other devices, the duplicate keywords could be restored from the originals and come back like a bad penny. It is futile to try to fight them in Photos.

After I discovered inconsistent Aperture keywords in my Photos libraries, I used exiftool to remove all keywords from the original image files. And then I migrated the Aperture Libraries again.

Projects are are bound to the library, where you created them. Apple Script cannot access the projects at all. And projects do not sync with iCloud Photos between libraries. If it were possible to access the projects in the libraries in a safe and supported way, the Power Photos team would have found a solutions, I am sure.

This has not changed with Photos 4.0. What has changed, is that most projects are no longer supported. Apple stopped the support for print projects and removed the tools to create native Apple Books, cards, calendars.

If you want to save your projects, export all print projects as PDF previews of the project, so you can have them printed somewhere else, or can at least view the PDF version.

If you want to be able to edit book projects, or calendar projects like you can do in Photos 3.0 on High Sierra or on earlier versions, I would keep a bootable clone of your Mac with macOS 10.13 or earlier, and a Photos 3.0 version of your libraries with the projects on an external drive as a backup.

After upgrading to Photos 4.0 on Mojave you can only create print projects in Photos with third-party project extensions. There is nothing you can do with your current books other than save the PDF version or convert them using third-party extensions and work with them online.