I think there is some confusion here.
I do not believe there is any limit on the how many Apple iD's you can create, or how many Apple ID's can be created (on device or other wise).
1) In general, you can only use one Apple ID at a time as the main Apple ID on an iPad. You can use different Apple ID's to log into different Apple Services (Music, Family Sharing, iCloud Drive, etc) on the iPad, but it is not recommended. It causes a lot of confusion and can result in buying products from different services that can not be consolidated.
2) You only get one opportunity to create an Apple ID directly on the iPad. That is when you set it up. There should be no limit to how many Apple ID's you can create up this way, but you have to start from scratch on the iPad each time. If there is a hard limit, it would be very large, as it would efectivly limit the number of times you could sell or give away the iPad. I can't imagine Apple doing that.
3) Here is Apple's support article on creating Apple IDs. You'll notice the last one is how to do so on the web.
I do not recommend this. While you might make it work for a while, it will only cause problems down the road. See point 1.
With your Apple Account, you can access all Apple devices and services â such as iCloud, the App Store, and more.
support.apple.com
3) What does have have a limit is how many devices you can have associated with the same service using the same Apple ID. For instance, you can only have a total of 10 devices (5 computers) signed into Apple Music using the same ID. If you try to do this you will get a warning. It is possible to go to your Apple ID settings and disassociate devices/computers, so this is not some hard/ultimate limit to your ownership of devices. It is probably there to prevent unreasonable sharing of services.
None of this has anything to do with your current problem, as far as I can tell. You ultimately need to get the original owner to release the iPad from his account.
I'm too lazy to read through this entire thread and double check what has/has not been tried and shared, so if this is a duplicate, I'm sorry. But you need follow the part of this support article under
If you no longer have your iPhone or iPad
Use these steps to transfer information to your new device and, if applicable, remove your personal information from your old device.
support.apple.com
You'll need the co-operation of the original owner. If you tried this once and it did not work, all I can suggest is trying again. Until the iPad is released from their account, all other efforts are pointless. All of them. No clever workarounds. No special tools available. At least not officially. No, I don't know of any special backdoor unofficial methods.
It is possible that the original owner, provided they are registered with Apple, can contact tech support and get instructions/help doing this.