Yes, the patches are cumulative, but I believe they are only applicable to a specific year's release, i.e. the final patch for 2018 will be OK for BIAB2018 but 2019-2021 patches will not.
That is correct. The latest patch for Windows 2018 is build 520
HERE.
Bob, if you would like, you could contact us and we can confirm the best way to re-install for you. Also, if you plan to upgrade to the 2021 version, if you purchase the UltraPAK or higher then you would not need to re-install your 2018 version. We would be happy to discuss your options with you.