If you think you can benefit from assigning targets, take charge! Before beginning a boss, assign a healer to each tank and put the rest on "raid heals". Your teammates might grumble, but when bosses start dying they will take notice, and you can start doing more involved tactics. Many encounters, particularly ones with healing complexities like Chimaeron/Baleroc or range issues that make it difficult to cover the whole raid, are absolute hell if the healers don't know who is ultimately responsible for whom.

Overhealing is sadly endemic to being a HoT-heavy class. However, anything you can do to heal more efficiently will improve your longevity and make sure you still have some in the tank when it counts.

First off, as a druid you are generally going to top the healing meters in any fight that involves raid damage because of the efficiency of using HoTs to top people off (unless your other healers are trying to snipe you), and because of the massive heals done by tranquility (if you pop it during periods of severe raid damage, it's not uncommon for it to do 15% of your total healing for a fight). If you're topping the meters on Baleroc then yes, you might have a problem.

Overhealing isn't an issue unless you're going OOM, or if it's indicative of people trying to snipe heals.

As for getting your raid to accept healer assignments this sounds like a basic one to me: the next time your raid leader says for healers to organize heals, be proactive and assign roles. If you wipe, call the person to task who failed at healing their assignment: if the healers have a single competitive bone in their body they'll start following the assignments to make sure that their assignment is done properly next time.