was disappointed that these cannot be sorted by date. If I were to insert a date prefix in the event names (e.g. 15-09-13 for today, or just 15-09), would that solve the problem?
No. Wait until 9/30 when El Capitan is released with the new version of Photos. It will have a number of missing features released as can be seen from the following paragraph in this Apple webpage: Apple OS X El Capitan.
Photos has been fine-tuned to make it even easier to manage your library. Now you can add a location to a single image or to an entire Moment. Naming your favorite people in Faces is faster with a streamlined workflow. You can also sort your albums — and the contents inside them — by date, title, and more.
Keep telling Apple what missing features you'd like restored or new features added in Photos via Apple-Photos Feedback.The feedback is working.
2) When I first looked at Photos, there was no batch change function. This was a surprising omission. Is there one now?
Not yet. However, there are a number of Applescripts in the Photos for Mac User Tips that will allow you to batch change titles and descriptions.
3) Again, when I tried it as a beta, the 'Moments' was a mess.
Have you tried repairing the library according to this Apple document: Repair Your Library?
Moments in Photos are the new Events, i.e. groupings of photos sorted by date taken.
When the iPhoto Library was first migrated to Photos there is a folder created in the sidebar titled iPhoto Events and all migrated iPhoto Events (which are now Moments) are represented by an album in that folder. To open the sidebar if it's not already open use the Option+Command+S key combination.
NOTE: It's been reported by several users that if the Event albums are moved out of the iPhoto Library folder in the sidebar they may disappear. It's not widespread but several users have reported that issue. Therefore, if you want to assure that you keep those Event albums don't move them outside the iPhoto Events folder.
There's a way to simulate events in Photos.
When new photos are imported into the Photos library go to the Last Import smart album, select all the photos and use the File ➙ New Album menu option or use the key combination Command+N. Name it as desired. It will appear just above the iPhoto Events folder where you can drag it into the iPhoto Events folder
When you click on the iPhoto Events folder you'll get a simulated iPhoto Events window.
The downside to the simulation is that the Albums/Events can only be sorted automatically by Title. But they can also be sorted manually, either in the sidebar or in the folder's window at the right. This will change with the release of Photos 2 with El Capitan this fall. That will allow the sorting by date or title, ascending or descending.