Hi there, I'm Jack, and I hope that I can offer a little bit of help to you.
In terms of your DK, you should be able to quite comfortably do any amount of AoE that you want, but if you think you're over-aggroing too much, or you're worried that you might be shouted at, or whatever, for what you do in dungeons, just attack the tanks current target. Pretty much for all classes, the tanks current target will be the one he or she has the highest threat against. Just target the tank (or in your case, click your focus frame), and hit your 'Target target's target' (that's a mouthful), which is "F" by default, and begin carnage with an oversized axe against it.
GRID versus HealBot is a discussion (or an argument) I often have with guild mates. My GM swears by HealBot, and I swear equally by GRID. In my personal point of view, I prefer using GRID because of its variety of styles and customization options, and its combination with Clique makes your healing your own; the key/mouse combinations you use can be configured with a GRID and Clique combo. While this is probably also attainable with HealBot, you should do whatever feels most comfortable to you, and what helps YOU do what you need to do. If there's one thing I've learned playing WoW, its that people do not often know what they're talking about.
As for tanking, I have not personally had enough experience with tanking, I much prefer healing. Hopefully some other kind soul can save you on this point. Good luck, and hope I helped,