. My old titles have thankfully migrated to Photos but now I can no longer add metadata such as a title or keywords to events, but only on individual photos.
This is not possible, because their are no events in Photos. Events have been replaced by Moments, that are created automatically based on a similar time and location. These "Moments" are named automatically.
What you are seeing in the folder "IPhoto Events" is a fake. Photos created an album for each of your previous events to save the name and the set of photos in the event. But you cannot add additional tags to albums.
You can add keywords to all photos at once if you select them and add the keyword from the keywords browser.
But there is no batch change command to change all titles or captions in Photos.
We wrote a few Apple Scripts to help with batch editing, see the User Tips page: Photos for Mac User Tips
For example:
Photos for Mac: Batch Changing the Titles to the Filename
Photos for Mac: Batch Change Title to Capture Date-Description-Padded Sequential Number
Photos for Mac: Batch Change the Descriptions
The scripts are best applied, when browsing the photos in the "All Photos" album. And not too many photos at once, because Photos will frequently have timeout errors.