But I probably want my first line to actually break follow and send the hunter pet into attack, so this would move down to the second line, and everything else would be bumped down a line? And then, of course, there's selecting the proper way to assist my master toon (/assist playername? @party1target? How should I code this?)
First off, don't put follow breaks in your DPS key. It will only come back to bite you later. Learn to micromanage and use additional keys for things like follow, break follow, buffing, etc. DPS keys should ONLY be for DPSing and IWT during combat {for melee toons only}. Most people use the arrow keys for slave movement and breaking follow. Start with that.

Second, you can't macro movement. This means there is no "moving down." You can tack /petattack in there if you like, but it will still go off at the same time as your first spell and it's likely your first spell will land before your pet even reaches the target. If you want the pet to get there first, you're going to need your petattack on a separate key and you'll have to wait for the pet to engage before your team engages.

Third, assist is managed in ISBoxer, you don't write an assist command anywhere in the macros. It should already be built in to the ISBoxer keymap. Adding additional assists to the ingame macros can lead to conflicts.


And what about other things like "/stopmacro [channeling]" and
/stopmacro [@target,noexists] [noharm] [dead] [channeling]
/script UIErrorsFrame:UnregisterEvent("UI_ERROR_MESSAGE");
/console Sound_EnableSFX 0

what is all this stuff? Some macro examples I'm looking at have it, others don't. What does it do? Do I need it?
http://wowpedia.org/Macro_conditionals
http://wowpedia.org/Macro_API


Second line: Not sure I really need a stun/slow/snare, so I could probably remove this, but for the sake of my education, what does the "reset = 0.3 0,0,0," portion do? How do castsequence macros using this reset differ from cast sequence macros using reset=combat or reset = ##, something that looks like this:

Code:
/castsequence reset=combat steady shot, steady shot, steady shot, steady shot, steady shot, arcane shot
http://wowpedia.org/Castsequence

This is a reset=#. In this case, reset is using an idle timer of a third of a second {0.3}, so the macro won't reset until there is at least 1/3rd of a second between button presses OR it reaches the end of the line in the macro. The 0's are not part of the reset, they are equipment slots used to put in a delay so the macro has a chance to try the spell on the next line in the macro.


Third and Fourth Line: Arcane shot. Great, I can emulate this line for all my instant casts and the spells that require some channeling time. But, again, I'm assuming with each spell that has a different cast time (2.7 secs for Starfire vs. 2 secs for Wrath, for example) I would need to alter the reset portion somehow?
Cast time is irrelevant. When building macros the game simply says

Button Press
What should I cast?
Am I already casting something?
Is there a conditional to consider? {nochanneling, dead, harm...}

If the answer is no to all of that, it casts the spell. If the answer is yes, it throws an error and doesn't cast the spell until you press the button again and the above comes up all clear. The Reset doesn't care anything at all about cast times, it only cares about the conditional it is assigned. In the case of reset=#, reset only cares when the last time you pushed the button was.

Your toons are just going to keep running their personal sequences with no regard for what any other toon's sequence is doing. You don't need to and shouldn't attempt to "time out" the macros among your team to make the cast times match. It is futile because of latency, haste etc, and the attempt will only ruin your DPS/"rotation." Build everyone proper macros for their class/spec and don't worry about timing it perfectly, it simply won't ever happen - even on teams where everyone is the exact same class and spec.



Last Lines: Whoops, totally lost me. Why is that stuff at the end of that macro and at the beginning of other macros I see?
/startattack just turns on autoattack
/target [noharm][dead] says "target an enemy if my current target can't be harmed or is dead"