Disclaimer: All of my answer ASSUME Ich is in the display tank.
1) Where do I go from here?
I would watch your fish closely. I would get a QT tank up and running NOW. This is in case you have to more ALL the fish out of the display tank and treat them with copper.
2) Do you think it was Ich? Parasites? Something else? We may never know?
Again, I can't be sure it was Ich, but if white spots like salt are on the fish, this is a sign of ich, it is also a sign of Marine Velvet, but Velvet has a sheen to it.
It is much more aggressive and contagious than Ich, the time you actually see MV on a fish, it is too late.
3) If I can't feed Caulerpa, and it tends to go sexual...what do you recommend for sumps? I have some Chaeto, but it breaks apart and gets in to my third chamber or hung up on the sponge I put to keep it out of the third chamber... am I doing something wrong?!
I use Chaeto, also mexican culupera doesnt go sexual
4) What about my other fish? Keep an eye out? Remove and treat with Hypo? Prazipro? Without knowing what happened, I don't want to cause undue stress...
Since Ich is now in the main tank, the ONLY way to eradicate it is to remove ALL fish and let the tank go fallow for a minimum of 8 weeks. Yes, that is correct. 8 wks.
5) Could the white specks be anything other than ich?
Yes, could be Marine Velvet, and the ONLY treatment that kills MV is copper.. Hyposalnity will NOT touch it.
6) Could the Ich have come from the live rock?
the rule of thumb is this: ANYTHING wet can carry parasites, even inverts...however, for Ich to survive it needs a host (that is, a fish)..you remove the fish, you kill the parasites, see the answer to question 4.
I will also add this...for ANY tang (and for that matter clownfish), I recommend to treat with copper only before putting into the main tank..dont bother with hypo..
the reason is tangs are much more susceptible to ich than most other fish...if i would to put a tang in my main tank, I would QT any tang for a min. of 8 wks with 2 of those 8 weeks dosing with copper treatment..just use cupramine and follow directions on the bottle...keep in mind a side effect of copper dosing is fish can stop eating...while you are getting that bottle a cupramine, I would also put up a bottle of Prazipro..this will treat flukes and worms..