Casting Arcane Shot should start your autoshot on its own, I believe. But I don't think the redundant statement hurts either, and it may handle cases where Arcane Shot misses or cannot be cast (mob in melee range).

If your main dies and you don't mind not moving, you can keep using that account to target things. This is often okay for PVE.

In the long run, I strongly recommend using keystrings. Assisting an explicit target is good for getting you up and leveling fast, but it's not ideal in the long run. You can set it up so a given keypress shows up as a different thing depending on which account was dominant when you hit it, meaning you can do something like:

/stopmacro [nomod]
/assist [mod:alt,mod:shift,nomod:ctrl,target=CharA][mod:alt,mod:shift,mod:ctrl,target=CharB]

Or something like that. I haven't tinkered with that one in a while but I believe this is roughly how it goes. You then stick it on a button, get its ID, hide the bar, and invoke a line like:

/click MacaroonButton37

in place of your original /assist line. The keymaps can be a pain to set up initially, and you may have to redo them a few times to get the bugs out, but you only have to do it one time and it's well worth it.

I actually use two versions of this (when I'm not doing quick-and-dirty macros to get leveling in a hurry), one to set my target pointer (not necessarily the actual target) to the current main and another to assist the current main. It simplifies macro creation and keeps you under the 255 char limit when using FTL modifiers.