I believe this probably is Aeromonas hydrophila rather than Aeromonas salmonicida.
The latter is more commonly the pathogen that causes the illness known as Furunculosis and it occurs mostly in cold water fish, though it can occur in tropical fish. I believe the ulcerations in Aeromonas s. would develop quicker and be more dramatic in appearance.
At any rate, Aeromonas h. can be the culprit in hemorrhagic septicemia, which I think this probably is. There are other pathogens that can cause septicemia, but often it is Aeromonas.
A cellulitis, or deep tissue infection, can occur with septicemia due to Aeromonas, and the infection can eventually extend to the surface of the skin and cause a pimple-like boil (furuncle) or ulcer to form. The red points or spots and then splotchy areas can certainly be both a symptom of septicemia and also deep tissue infection. Of course the infection can also be wreaking havoc with the internal organs as well, which can lead to dropsy due to organ failure.
* I have successfully treated hemorrhagic septicemia with Minocycline, or Maracyn 2, but I think Chloramphenicol, as suggested by Cerianthus and Sub Rosa, would be the most likely "magic bullet" for this outbreak.
* If you can, I would take Sub Rosa's offer and get him to send you some Chloramphenicol ASAP, if you can't get it locally.
At this point I don't know if it could get the Chloramphenicol to you fast enough to turn things around, unless you could ask a local vet or get Sub Rosa's meds overnighted to you.
(I tried to get it for an African Dwarf frog with Chytrid and my vet was willing to help, but she didn't have any of it . She gave me a script to have it compounded at a local pharmacy but they wanted $78 to do a very small quantity.)
* My next choice would be Furan 2 and Kanamycin together. Oxytetracycline is good. Maracyn 2 is good.
* Since you've been using the E.M. Erythromycin you've continued to have losses; I don't know if this is going to stop the progression of this through your tank occupants. If you can't get the other meds mentioned, you might try to mix some in food to get it into them internally to try to help the effectiveness.
Aeromonas bacteria are almost always present in most tanks and usually don't cause an outbreak until something weakens the immune system of the fish.
Your tank has been cycled and stable, your parameters are good. Do you use a liquid test kit? Strips may be giving inaccurate readings of your parameters.
Common stressors are unstable temperature, overcrowding, handling stress, poor filtration, high ammonia, nitrite, nitrate levels, unstable pH, high amounts of organic decomposition (often in the gravel or filters). Parasites can be a factor.
Your tank is 12 gallons, I think you said in one of the other threads, and the parameters are more prone to sudden swings in smaller tanks, (speaking from experience with a 10 gallon), is this a possibility?
It sounds like you have a good routine of keeping the water quality good, but do you vacuum the substrate regularly to remove decaying mulm, debris, leftover food? Decaying food can be a medium for bacterial growth and when eaten may be a factor.
I would try to keep the water pristine, and I would use Prime to help detoxify any traces of ammonia, nitrite and nitrate during the treatment period. Improving oxygen levels and circulation is important for prevention and recovery so you might add an airstone if possible.
Can you tell us about your gh/kh and ph in the tank just before water changes, and what those parameters are in the water straight from the tap? I think I read that your habit is to change the water once a week at about 50%, is that correct? Unstable ph can be a factor, but more significantly swings in TDS.
Forgive me for asking questions you may have answered in other threads, and I don't mean to be asking questions that sound like I think you're inexperienced... I'm just throwing things out there to try to figure this out, I really want to help. I'm sorry I'm so late getting in on this with you, but things have been really hectic at my house for quite a while.
I hope your fish get better... I'm so sorry you're going through this difficulty and for your losses... I know it hurts to lose these lovely little creatures, and you are trying so hard to save them.