It's hard to tell if this is just a tongue-in-cheek, or not.
If not, you are making assumptions... big ones, that I didn't post.
1) Yes, the cells for the 12 volt can be in the main pack housing.
2) That has absolutely nothing to do with HV, parking brakes, or the rest.
3) Coming out of the pack (and all the climate controlled cells) would be two HV cables to the inverter, both on a relay internal to the pack. That relay is powered by 12 volts, and is normally open (NO). That is what exists today.
4) In addition, two non-switched LV - 12 volt cables would ALSO come out of the pack. That is different than today.
5) There would be no external 12 volt battery, no lead acid, and no changes to the operation of the car.
6) This 12 volt would likely last as long as the main traction battery cells.
7) If the pack cells are getting some Temperture Management System (TMS) love, then so will the 12 volt portion of the pack. The amount of extra energy required for TMS duties of the additional 16-40 cells would be difficult to measure... that's how small it would be extra in relation to the 7104 cells already in the pack. I calculate less than 0.5% additional energy in exchange for a lifelong 12 volt battery.
It seems I had posted a little bit too quickly… I did not think about putting the 12V inside the main battery.
Then I agree with all your points except the last two.
For (6) I'm going to assume we have separate cells (of the same chemistry) inside the main pack wired to make 12V.
In that case, the 12V cells will have a very different usage compared to the HV cells. When the car is off, the 12V cells will be cycled a few times a day, while the HV do not get used (just to top off the 12V.) When the car is running or charging, the 12V cells are maintained at their ideal SOC while the HV cells get cycled. So the more you use the car, the less the 12V cells gets used.
I can't say if it they will last more or less than the HV pack, but I don't believe it will have the same life. Either way, if it get a good TMS, I agree it will probably last longer than the actual lead-acid.
Regarding (7), it is my understanding that the batteries Tesla uses have different acceptable temperature range depending on whether they are not used, charging or discharging. If I understand it correctly, they have a very large temperature range when not used, a somewhat smaller temperature range when discharging and an even more restricted range when charging. Lead-acid accept a much wider range of temperatures, and it is not dependent on the whether or not the battery is in use or not. The temperature just changes the voltage setpoints for the battery charging.
In most situations, when off and not charging, the MS does not actually run the TMS because the cells are not used and thus are in the wide range of acceptable temperature. (That's why the vampire drain is almost the same for everyone. It does not depend on the external temperature.)
If Tesla were to replace the lead-acid by a battery using the same chemistry as the actual HV pack, the TMS would have to run more often to keep these few cells in the acceptable temperature range for charging/discharging. In that case, the difference would not be 0.5%, it would be 0 versus whatever is required to keep the temperature of these cells acceptable.