Well, Mike, as you seem to be addressing me, I'll respond.
Just accept that slow was in common use as a flat adverb
Ah, on the principle that usage remains valid if it was valid in the past. That is known as the "I shall return incontinent" argument (Othello). "If it was ok for Shakespeare, it's ok for me"
If you accept "as it were", why not accept "drive slower"?
Er ... because they're different. That do you?
There seem to be two main reactions to the existence of alternative forms:
(1) declare one version the only correct one (invent a rule)
Mike, one does not "invent" rules in language. Language is, has always been, absolutely full of rules. Either you retain rules or you change them. No need for invention.
Consider whether it is your manner of driving or the car that is slower.
This is very schoolmasterly, Mike. My car is pretty slow, but that has nothing to do with my driving. Sometimes I drive slowly, sometimes I try to drive fast. I certainly would not have a "slow manner of driving", that would sound really weird. I might have a manner of driving slowly, though that sounds a bit odd to me too.
Consider whether you would say "I inserted it slow"?
Nah, that'd be rubbish, innit.
Consider who has the authority to change the rules.
Who on earth started talking about "authority"? Really, Mike, your imperatives are not just schoolmasterly, they're oberlehrerhaft. I gave an example of the loss of the adverb, particularly in AE. I did not presume authority to tell others how they should speak. I did give an example of one of my own faulty utterances, and I hope you will allow me the authority to decide for myself how I speak ;-)