for macro's heres some useful guides, and to answer your questions, yes you can tell your group who you're mezzing, no it wont happen right as you hit the mez skill, but u can set a shortcut next to the skill, thats what i do.
http://forums.lotro.com/showthread.php?t=144140 http://forums.lotro.com/showthread.php?p=426746
i dont know of a hotkey for assist, but you can enable "skill target fowarding" under combat options which is extremely useful. if you select someone on your side (whether in group or not) and you use an attack skill it will attack their target, if you select an enemy and use a healing or buffing skill it will heal/buff their target.
yes theres healing aggro, but as you go up levels the healing classes get skills/traits to lower healing aggro so that a skilled healer shouldnt get much aggro at all.
you can see the circle around the mob's portrait that says how hard it is to kill, heres a thread on it
http://forums.lotro.com/showthread.p...=1#post4153573