I understand exactly what you are saying, but not everyone is Riseball. Not everyone is Rick Pauly. For every parent out there that understands and can see the adjustments needed there are 50-100 more that a overbearing "know it alls" that just want to live through their DD's success. I guess my experiences around the ball parks have shown that more often than not mom & dad are being meddlesome not because they are knowledgeable, but because they "played some ball back in the day". I think that you would also be surprised at how many people on this forum either don't understand pitching mechanics, or have studied them but don't have the eye to see the flaws.
I guess what I am saying(and take this as a compliment) is that you are probably the exception, and not the rule...
To your point a parent/outside coach needs to know their limitations. It is one thing for a player to seek out guidance, it is completely different for a parent to run to the dugout before and after every one of Suzy's at bats and demonstrate proper hitting mechanics. When there are not in the dugout with me I am available for my DD, my niece, and my students but they all know that they need to come to me. Make yourself available but do not intrude. It is up to the player to work out the protocol with the coach for any interaction. When I am in a recruiting role I was much more concerned about the parent engaging the player than the player engaging the parent. When the time comes addressing an issue with the player is much easier than one with a parent.