With respect to #1 and #2, see:
How to create a new Apple Account - Apple Support
Separating out your old photos (How to separate merged photo albums when … - Apple Community) may be a bit trickier. If you are currently using iCloud Photos, the master copies of the photos would remain with the old Apple ID.
Presumably you could turn off iCloud Photos on one device only (the one that will be getting a new Apple ID) and then change your Apple ID.. That device might still have copies of all the photos and videos (which are no longer synchronized with the other device), but if you had been using "Optimize iPhone Storage", I'm not sure that they would be full-size originals.
How to turn off iCloud Photos - Apple Support
Once you turned off iCloud Photos (on one phone ONLY) it presumably would be safe to start deleting the photos from it that you did not want to keep on it.
I would be very careful about deletions, since as long as the two phones are synchronized using iCloud Photos for the same Apple ID, any deletion on either phone will be a request to delete the photo on both phones. Likewise, if you are not using iCloud Photos, but are doing iCloud backups that include photos instead, as long as you are on the same Apple ID, you could be making "competing backups".
