Manipulating image files outside of Lightroom often causes problems. Your image files are referenced in Lightroom by they path and file names. Moving or deleting them at the OS level leaves LR "confused" as it no longer knows where the files are.
In getting the files onto your laptop, you should have either copied both the images and the catalog, or "exported as catalog" to the laptop.
Since you deleted the original files referenced on the desktop, you need to delete them from the desktop's LR catalog also. Then you should be able to "import from catalog".
Thanks Joe. In trying to solve this I did do as you suggested, using "remove" to get lightroom (on the desktop machine) to forget about the original photos. I still wasn't able to import from catalog the edited images from my laptop. I also went ahead and deleted the photos from the OS too (not that that should make a difference to LR) just in case, and still no ability to import from catalog.
Thanks for the advice on how I should have originally transferred them to my laptop. Will stick to that workflow in future. In the meantime, for these photos I still can't bring the catalog with the edited images back into LR on my desktop. The dialog box still refers to 193 "existing photos that are missing." I've removed the original, "existing" photos. What could it be referencing?
I didn't solve the problem but I did find an acceptable workaround. I restored the images I had deleted back to the original folder, then copied (using file manager) the .xmp files from the edited images on the laptop, pasted them into the original folder on the desktop computer, and then told LR to overwrite the original metadata. It's not the same as importing from the catalog, but it's good enough for now.