I am a grammar pedant, and I understand the use of 'myself' as a reflexive pronoun.
However, it is perfectly legitimate to also use it to add emphasis, to make a note where something is unusual or different, or alongside the noun to which it refers (also for emphasis).
Examples:
I am sure the door was locked - I locked it *myself
*
We talked to the teacher himself about the situation.
Whether or not Charles needed to do it in his speech is arguable. It does soften the tone & delivery, and I don't consider it a grammatical error in the way so many do here.
(The mention of William's 'by Catherine & I' is something I have noticed too & that's definitely grating).
Finally, I'm Irish, and in Hiberno-English, 'myself' is used liberally and frequently and is not considered incorrect, tho maybe more informal. 'Is it yourself?' [as a greeting on the phone or when you open the door]; 'Did you have a nice Christmas yourself?'; 'Great. Yourself?' as a response to 'how are you?'
The example of the insurance agent saying 'is it for yourself' is not only common here, it's considered more polite, than saying 'is it for you'. Just less direct & harsh.
While I think good grammar is important, so is using language responsively & contextually, and it means, that even with grammar, sometimes rigidity is incorrect.