Well, I'm 71 years old, so I don't know how much good I can do you. And the other problem is that I'm on Windows. But here's what I would do:
1. Start Lightroom, and then go to File/New and create a new empty catalog.
2. Using that new blank catalog go to File/Import from Another Catalog.
3. Then choose each of your other catalogs.
Now, understand that I have never done this. So I'm not sure how it will handle your folder structures and duplicates. But if the new catalog isn't satisfactory you will still have all of the other catalogs that you can return to and try something different. I have two catalogs. One contains all of my images that I want to keep. The other one is a test catalog that I use exclusively for experimenting. I just like to do it that way. Anyway, give it a try and see how things turn out.
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Hi rushbo. I am of the opinion that it is much better to save as much information with the files. Use Lightroom to organize and process the images, but keep the data with the images when possible. (There are tradeoffs to this approach, mostly speed and complexity, but it means that there is one place to look for information about a file, even if you have multiple catalogs).
If you agree with this, first read What Is Not Included In Lightroom XMP Files | Lightroom Fanatic to see what data is not saved.
Then I would go from the oldest to the newest catalog, and for each turn on "Catalog settings:automatically write changes into xmp" and then save the metadata for all the files in the catalog (Select All; Save Metadata). You might also want to choose "Delete Rejected Files" to get rid of them permanently.
Do this for each catalog, and at the end every image will have the settings from the latest catalog (earlier values will be overwritten).
Then create a new catalog and import the files. This will bring in every image in your directory tree (wherever you store images on your disk). It might even discover some images that aren't in your catalogs. And each image will have the latest data. Finally archive your old catalogs "just in case".
This process can be slow, so put on a movie...
PS--there is no way to deal with the name changes unless you do it by hand for each catalog before you start any of the processes discussed here. Of course, if you use my method, then any catalogs with out-of-date file names will just have an error on saving the data, and your final catalog will have correct names.