I have a regular retail sailfish Google Pixel (i.e. NOT a Verizon one - from EE in the UK, code _pixel_phone_black_32gb_p), and have tried all the methods in this thread to unlock the boot-loader without success . I was under the impression that the non-Verizon pixels were "less" locked and easier to unlock, but is this no longer the case?
I can't answer all your questions, but some. As far as I know, it's more the reverse - the ONLY edition of any Google Pixel that can absolutely be counted on to be able to unlock the bootloader is a Google Edition, so that would mean not buying one through any carrier.
Finally, is there any way to sideload a newer Android, or does that only work with the archives on the site: https://developers.google.com/android/ota#sailfish ?
Until or unless you can unlock the bootloader, you can only use official releases that are aleady available for your phone. The last official update was "10.0.0 (QP1A.191005.007.A3, Dec 2019)". In order to do anything else like flash someone else's ROM that might be based on a newer version of Android, you have to unlock the bootloader first. If you can't do that, then there's nothing you can do.