Following some trial and error, and additional reasoning, I can partially answer my own question:
(1) The events from a local calendar, on an iPhone ("On This Phone", I think it is referred to), are not backed up to the calendar-specfic area in iCloud. Here:
iCloud.com > Settings > (Advanced section) > Restore Calendars and Reminders
Although it doesn't say so explicitly, it makes sense that these iCloud calendar backups cover a user's iCloud calendars. (Plus, my attempt to restore such backups with a hope to see pre-merged, local calendars, failed).
Screen shot from iCloud.com:
(2) The iPhone backup (the complete backup of the whole phone), which in my case was being backed up to iCloud (rather than iTunes), would, I presume, include the local calendars and their events.
However, by time I'd got to looking at this, my oldest 'complete' iCloud backup was just a day old... which was after I merged the local calendars into iCloud calendars.
Turns out I solved the issue by just wiping my phone and restoring it from that same Sept 27 iCloud backup again. It had the photos in the backup after all. As to why it didn't download them to my phone when I restored from that backup a month ago, I'll probably never know.
Before I actually wiped/restored, I wanted to make sure that the backup actually had the photos in it. Of course Apple doesn't let you see the data in your backups, so I had to download a 3rd party backup viewer on my mac to look at the pictures in the backup file. Once I saw the photos were really there in iCloud, I decided to just restore from that backup and see what happens. It ended up working. Weird.
Best Answer
You can see your available backups in iCloud when you go in your iPhone to Settings > your name > iCloud > Manage Storage > Backups. That’s all you have in iCloud.
If none of these backups store the data you need, then the only hope can be in the Time Machine backup before the reinstallation in case you backed up your iPhone in iTunes/Music app. In that case you may want to restore content of this directory while iTunes is not running:
/Users/[your user]/Library/Application Support/Mobile Sync
Once done, you can start iTunes and see restored backups in Settings and the tab with devices.