Please me help understand macro commands

I am a Holy Paladin (off spec) and sometimes serve as my raid’s tank healer. I usually always have the main tank on focus.

I would like to modify a macro to allow me to add a [@mouseover] so that if a particular raid member is in trouble I can quickly target them directly, otherwise just heal my focused tank. I wanted to use one of these macros, but I don’t understand the macros well enough to know what they are doing differently. So I don’t know how to modify them to accomplish my goal.

These are the macros:

#showtooltip Judgment
/targetenemy [noharm][dead]
/castsequence reset=0.3 0,0,0,Light of Dawn
/castsequence reset=0.3 0,0,Crusader Strike
/castsequence reset=0.3 0,Judgment
/castsequence [@targettarget] reset=combat/15 Holy Shock,Holy Shock,Holy Shock,Holy Radiance

#showtooltip Judgment
/targetenemy [noharm][dead]
/castsequence reset=combat/5 !Seal of Insight,null
/castsequence [@focus,help,nodead][@player]reset=combat/5 Beacon of Light,null
/castsequence reset=0.3 0,0,0,Holy Prism
/castsequence reset=0.3 0,0,Divine Plea
/castsequence reset=0.3 0,Judgment
/castsequence reset=5 Holy Shock,Holy Radiance,Light of Dawn

First question, what does the /targetenemy [noharm][dead] command accomplish and why is it at the start of both macros? Is this line applicable to raid/dungeon healing?

Second, the last line in both macros is almost the same except for the [@targettarget] modifier in the first macro. Would this cause the first macro to always target the tank that has agro on the boss?

Third, does the line in the second macro, /castsequence [@focus,help,nodead][@player]reset=combat/5 Beacon of Light,null only apply to that line or all subsequent lines in the macro?

Thanks in advance for any help.