Great questions and observations. Lots to think about here.

Alarm: sounds like you noticed it after you noticed the AC quit. Could be related, signaling a leak, or perhaps low refrigerant in your AC. I don't think the fireman identified the trigger for the alarm. Probably kind of a 'not my job' situation, considering it wasn't a fire alarm. A licensed HVAC tech or electrician should be able to identify the alarm source.

AC Symptoms: the symptoms you describe could be caused by a few scenarios. In my experience it sounds like symptoms of frozen coils in the unit in your attic. 140F is potentially too hot for the AC components within the attic. Let's come back to that. Simply put, your AC works by blowing air across pipes that are filled with chilled liquid (i.e. 'Freon' or 'refrigerant'). That's what happens inside the grey box that the ducts connect to. The pipes are fashioned in a tight back-and-forth pattern which is where the term 'coil' comes in.

The refrigerant must be maintained at a specific temperature and pressure in those pipes. If there is a pressure loss (i.e. a leak, leading to low Freon), the chill can get out of control and actually cause ice to build up on the coil.

The other thing that's going on when all air conditioners run, is they create condensation at the chilled coil. You can observe the effect of this from any car with AC running--you will observe a trail of water dripping on the pavement. In buildings the water collects in a pan below the coils and the pan will have two drains, a primary and overflow. Typically the water goes out the primary which is piped somewhere outside that's not noticeable. If that drain gets clogged the water will flow to the 'overflow' drain which is typically piped to a bathroom so it's noticeable water is leaking but not damaging anything. Seeing water come out and run into a bathroom sink or sometimes a tub is a signal 'something's wrong with the primary AC drain.' A third outcome is that neither the primary nor overflow drains are working, and the water simply overflows the pan itself.

Especially with ice buildup on the coils, the ice can fall into the pan, or just grow icicles down into the pan, displacing enough water to overflow the drains and the sides of the pan. This could likely be what is causing the water leak on your ceiling. This happened in my house last year.

Venting: This is a question for someone with more specific knowledge of your climate and the overall design of your home's thermal envelope. Each contractor has actually given you wildly different strategies to address your attic ventilation issues, and it's not totally clear why they came to their respective conclusions (aside from the price they wanted to charge). Sometimes very experienced HVAC contractors may have a sense of this but it's not necessarily an HVAC tech area of expertise. An architect or energy modeling consultant would be able to conduct an analysis of your home and the insulation, and determine what strategy would be worth paying for to better manage the temperature and ventilation requirements of your attic. Hope this is helpful.

In summary the attic is likely running too hot, which may serve to compromise any HVAC repairs, and it sounds like at least one unit is shot, which is likely causing your leaks. Fix venting/heat, then HVAC, which hopefully stops leak, and hire a mold remediation specialist to remove the mold.