Password rotation in this context is for the infrastructure, not normal user accounts.
There are other options for securing/managing infrastructure access (e.g. PKI, Hashicorp Vault), but if you're using passwords, it's a good idea to rotate them if only to encourage good practices around automation.
You're likely seeing that from a third-party seller, similar to Amazon's marketplace or ebay. At the least, I wouldn't expect the chair to be brand new at that price.
If you don't care about the managed Google Play store, you could always use Google's TestDPC app (or create your own) to create a work profile ("do-not-use-in-production" warnings notwithstanding): https://play.google.com/store/apps/details?id=com.afwsamples...
That said, it might be more straightforward to just use another user on your device
No, the check is done when the CA issues the cert. This allows you to change your CAA record without making your cert invalid (see also https://tools.ietf.org/html/rfc6844#page-2)