Hi Raj,
Welcome to Microsoft Forum and hope my test results could offer some help on your concerns.
1.Yes, currently the minimum number of major versions is 100 and there will pop out an error window giving you the available range. For the reference: Enable and configure versioning for a list or library - SharePoint (microsoft.com)
2.If the set number for the library version is changed from 300 to 100, the existing files which used to have the versions of 300 will be deleted to 100 and the older versions like 1.0 to 200.0 will be deleted and versions from 201.0 to 300.0 will be kept to meet the new rules on this library. Just like when you do another edition, the version history will keep versions from 202.0 to 301.0.
Hence, make sure to avoid any possible data loss before changing the number of version history. Also if you need some files to keep a larger version history but still want to set a smaller number in this library, a workaround I would recommend is to move some important files to another new library to keep some more data.
3.For the third concern, we are sorry to say the version history of a file is counted only after it is uploaded to a SharePoint site or OneDrive for the first time. The versions of a file saved in A site will no longer exist if it is uploaded to B site from a local/offline environment. Hope the section of When versions are created from this reference could offer a little help on this concern: How versioning works in lists and libraries - SharePoint (microsoft.com)
Do let me know if my explanation is not that clear to you or I could do better on this scenario, I will try my best to offer any help as I can.
Best Regards,
Mia
