Some of that is covered from a legal angle; they might be able to technically pull it off, but they'd also go to jail for doing so
You can have 2 trusted people be administrators of the 1P account. Then you create a shared vault for anyone who needs access to the master credentials and store them all in said shared vault. You can have multiple vaults with multiple different sets of credentials and access (Ex a vault for HR is going to have different creds and people than a vault for DevOps). Note: The list of administrators and the list of people with access to the shared vault doesn't need to be the same people.
If someone who's in the trust circle quits or gets fired, you have someone else remove their access (just like any other offboarding) and go change the applicable credentials.
As for insider threat, there isn't a lot you can do if you give someone access to the credentials. For the admin, that should be easy - certainly, you can find 2 people in an organization who can be trusted implicitly (even if one of them is required to be the CEO/founder). If you're worried that someone is going to go rogue, then don't give them access to the information in the first place.
There is some stuff that just doesn't need to be broadly shared...lets say master passwords for AWS accounts. Really very very very few people will ever need those - mostly for "Break glass in case of emergency" purposes, right? So your Director of DevOps has access and maybe one other person. If you can't trust that Director of DevOps to have those credentials.... well honestly your organization has bigger issues.
Not necessarily. You can set up things so that that requires two persons (https://en.wikipedia.org/wiki/Two-man_rule), or any 3 out of a group of 5, etc (https://en.wikipedia.org/wiki/Shamir%27s_secret_sharing)
RE: Sharing.
Nobody should be sharing a password manager account. If this is happening it usually means somebody doesn't want to pay for the seats to have individual accounts. You give each user their own account, and if you need more than one admin, you have more than one admin, but you shouldn't be sharing an admin account.
This would also apply if you're not talking about the pw manager's master password but something more like an AWS root account. That root account needs to exist, but its credentials should be a secret in the password manager vault that is shared with anyone who needs it (based on roles/access rules/principle of least privelege), and you should create additional administrative roles within the system you're securing instead of leveraging those root credentials (as much as possible) operationally.
RE: Bad actors
SSO is the answer here. You use a password manager with SSO integration, and you cancel the person's account the second they depart. At previous orgs they literally cancel the account while the person is still in the building and being walked down the hall to be informed that they're being asked to leave. It's definitely on the cruel side but if you're concerned or wanting to emphasize the security of the data first and foremost, that's what you do.
Both parts of the answer above involve $$$. Most services require paying for a higher tier if you want SSO integration (both on the side of the provider like Google Workspaces and the tool like 1Password), and both require paying for head count.
But this is the answer to "best practices". If there's a financial impediment that prevents you from doing these two things, then you can't use the "best practices". That's the unfortunate nature of the beast, and why there are so many online disagreements about the concept of an "SSO Tax".
For larger companies, you'd use CyberArk, so you can grant roles, get audit logs, use a HSM, etc.
AWS recommends actually throwing away the root key after you grant full access to an IAM user. In the rare case you need it, you can recover it via support.
But generally speaking a password manager with built in credential sharing is your best bet. In most cases the CEO would own that account, or in a good org ownership is shared and/or split amongst a few top execs.
But if you want a model to not follow, don't just share the AWS root key with all devs. That's what we did at reddit when we first started, before any best practices existed (And before IAM existed).
100% of the things done to access/modify our secrets & infrastructure are logged in our cloud ecosystem in a manner completely outside our control (AWS or Azure hold the log). We cannot delete or edit these. All we can do is read them and receive reports or alerts.
If someone with global admin decides they want to become DBA over a highly-sensitive SQL Server instance, nothing would actually stop them from acquiring this access. We aren't protecting nuclear material or other defense/life-safety-critical items. If someone wants to risk their career to do the wrong thing with our data (or our customers' data), then I say go for it. I will lose exactly zero sleep terminating someone who does something that stupid. Our policy isn't even explicit. This is one of those "if you have to ask, you shouldn't be working here" kinds of things.
Fear is a much more efficient tool than key splitting schemes, physical vaults, armed guards, etc.
Split the secret into 5 pieces and require any 2 pieces to recreate the master key. Give the five pieces to senior managers or technical resources.
Regenerate the secrets whenever someone leaves.
Store the encrypted secret in a folder where only the five users can access it, and access is logged to cybersecurity.
Or, print it and store it in a safe in an access controlled room.
The developer signing keys are set to expire after one year, while the QMSK [Qubes Master Signing Key] and RSKs [Release Signing Keys] have no expiration date. The QMSK was generated on and is kept only on a dedicated, air-gapped “vault” machine, and the private portion will (hopefully) never leave this isolated machine.
Alternatively one could use split-gpg, but it's probably a tiny bit less secure: https://www.qubes-os.org/doc/split-gpg/.
See also: https://github.com/QubesOS/qubes-issues/issues/2818
For ultimate offline secrets like HSM, CA, or crypto private keys, buy new printers, print the secrets as QR codes and keep them in safety deposit boxes at 2-3 different banks.
For passwords you don't want any one exec to have, use N-person keying by splitting 1 or multiple keys into pieces such that a quorum of 2,3, etc. are needed to utilize the secret.