Does matter whether the abusers are the owners themselves or merely representative agents of that the owners have hired?

I don't think that it does.

Per the definition, yes. A DMCA notice submitted by someone other than the copyright owner or someone acting on their behalf is invalid.

Also I could totally seeing this getting abused with competitors posting fake DMCA for one’s content to get their competition public domained.

That seems like you’d be committing perjury, which the victim could prove easily by showing there’s no authorization.

If you submit a DMCA notice without authorization, then it’s perjury.

If you submit one with authorization, then the authorizing party should put their IP as collateral.