I haven't had any issues using the iCloud photo library to commingle photos amongst iOS 8, 9.0 and 9.1

My safety valve is of course OS X Photos where I have point in time backups of the originals.

You don't need any software on the PC if you want to move your photos into the cloud before you do the wipe and downgrade. Also, you can use iTunes on the PC to make two backups of the 9.1 device. Assuming your downgrade goes badly, you can at least go back to where you are today with the photos on 9.1 again and re-do the backup or choose a different photo sync solution.

I haven't had any issues using the iCloud photo library to commingle photos amongst iOS 8, 9.0 and 9.1

1. Enable cloud library on the phone
2. Wait and check all is there at https://iCloud.com/
3. Restore to iOS 9.0
4. Sign in to iCloud and enable cloud library - download originals

My safety valve is of course OS X Photos where I have point in time backups of the originals.

You don't need any software on the PC if you want to move your photos into the cloud before you do the wipe and downgrade. Also, you can use iTunes on the PC to make two backups of the 9.1 device. Assuming your downgrade goes badly, you can at least go back to where you are today with the photos on 9.1 again and re-do the backup or choose a different photo sync solution.