snatch1eB
I would definitely contact seller for that. Warranty is one of the things why you get the new drive, at least that is critical to me. Return the drive if they would refuse to provide you with the warranty.
In case of HDD, you have 3 brands to work with Seagate, WD and Toshiba, they are all fine in terms of reliability, you need to look for the best $/TB ratio and warranty time. To keep your data safe, I would rather think to implement 3-2-1 backup rule. https://www.veeam.com/blog/321-backup-rule.html
As it was said, you may try unRAID which would handle even the different size drives.
As alternative, it looks like mdadm should fit your requirements. You can use smth like openmediavault for that.
For the shared storage, you may try to go with the Starwinds vsan, the system requirements are pretty low and you would be able to use the internal storage as shared one, so no need of the additional box. Also, it has guide with the steps, so shouldn’t be an issue: https://www.starwindsoftware.com/resource-library/starwind-virtual-san-vsan-free-configuration-guide-for-proxmox-virtual-environment-ve-kvm-vsan-deployed-as-a-controller-virtual-machine-cvm/
I believe you just got bad luck with those drives. You should be able to RMA those drives if they were bought as new. I had a few drives which failed in a few months, and some that are running fine for 5+ years.
Depends on the NAS. Some of them offer native cloud option or access via the VPN.
Recertified drives are the ones which were in use and were returned due to issues. They usually get fixed if required and sold again. They usually have the additional limited warranty, which makes them a good option for lower cost.