I agree on the need for a sprinkler installation code, and agree that it's a risky DIY project, just don't like restricting installation so a plumber can't do it. The pipe used in residential systems is a flame retardant PVC approved for the use. As for using PEX or PVC, it's another trade off. By the time the fire is big enough to bring down the drywall and expose the pipe (remember the sprinkler head is flowing), it will take a minute or so to burn through (longer if water is flowing, think boiling water in a Dixie cut on a open fire). By this time the occupants are out. But before this drywall collapse point, there is a lot of time and the fire is either mostly put out or the speed at which is spreads is reduced dramatically. This is another facet of the goals of the system. Is this a life safety system or a protect the property system? More choices, more opinions, more debate. Type of construction is a factor too. "Stick built" vs. "gusset plate trusses" affects how long until structural collapse. Ties back to the goals: Is this a get people out safely or a put out the fire 95% of the time system? Note that plumbers are skilled in installing PVC systems, and the "special" PVC used in residential fire systems installs exactly the same as "normal" PVC and is therefore within the plumbers skill set.
The how many heads needed is also "pondering point". If you legislate that the entire house must be done, the cost goes up quite a bit. If four heads can get more than 50% of the fires, that's not bad. I did my entire house except for my electronics area. A hard rule means that I would have to have done that too (or go back after the fact and defeat them). As for the "We are talking about lives here, so we must do the whole house to at least a 'protect the people' level" argument, well if that were true we'd also mandate driving only cars with full airbags, collision avoidance radar, and the like. All houses would be framed with steel. You see the problem, the cost of everything triples is this protect life
at all costs criteria is uniformly enforced.
Yes, it's a big decision to figure out how many and where to install heads, but this is no different than many safety vs. cost trade offs we make every day.
As for alarms, a residential system uses the existing cold water distribution to run the sprinklers. A commercial system has a dedicated set of pipes. This trade off makes the residential systems less expensive (and should mean that a plumber can install it). Unfortunately, it means that you can't look for flow in the pipes as an alarm trigger like you can with a commercial system. So the alarm aspects of a residential system fall back to the smoke detectors. We also see more cost trade-offs here. Less robust system in residential (can't detect water flow) but at a lower cost.
Come to think of it, smoke detectors are another area where we all make cost vs. safety trade offs. Most residential environments just have the smoke detectors as per code (in and near bedrooms, at least one per floor). I have additional detectors at my house, one in every room (such as the laundry, TV room and office). I also have heat detectors in the kitchen and garages. These are cheaper than any sprinkler system, yet it is not mandated that you have detectors in each room. For that mater, many residential alarm systems are wired so that you can't tell which detector is sounding (cost less to install that way). If the system can tell where the fire is and announce it, that good information for both the home owner and the fire department. So if you applied the same economic trade offs to sprinkler systems that we currently apply to smoke detectors, you would have certain areas with heads, others without.
All safety vs. cost decisions suck. And since you're drawing a line in the sand, there will be fierce discussions by passionate people about their views. I have my entire house sprinklered based on what I've seen. But I don't want to force my views on others, and vise-versa. If I were King of the Universe, I'd sprinkler the kitchen, laundry, furnace area, and a head in each hall by a bedroom. And of course you can add more as you would like. That in conjunction with smoke detectors isn't a bad imposition (in the King's opinion). But this is the real world, so we struggle with where the line is, draw it as best we can, accept the rules, and tractor on

.
Back to the OP's concerns, this all comes back to what's reasonable to require, what's reasonable to leave to the discretion of the home owner, and what's reasonable in terms of limits on who can do the work and how the work gets inspected.
Pete