[B10alia]

I can never get the PCV elbows to last more than a couple of months, regardless of where I buy them from. They all seem to have the exact same part number on them whether they're from NAPA, Autozone, Advance, whatever. They're pretty cheap, so if you haven't replaced yours in a while, I would look there first. Hard plastic vac lines can be replaced with hose.
I doubt it's the transmission speed sensor. That should also give you speedometer issues, as it's also the pickup for that.
I also have my doubts about the MAP sensor unless you're also experiencing more fuel consumption. Any failure of the MAP lines will cause a reduced vac signal to the computer, which translates to a "increase" in engine fuel demand, and a persistent rich condition. Personally, I have never heard of a MAP failing lean, but I'm sure it could happen.
Unintentional converter lock would would not explain the flare on startup, and I agree with you that it's extremely unlikely.

 

[B10alia]

Plug the vac lines to the evap canister and see if that changes it. IIRC they can be difficult to find and the price isn't nearly as trivial as the PCV elbow.

 

[B10alia]

It's pretty hard to hurt a 2.5, they're very durable engines. Maybe the torque method wasn't followed when the headgasket was installed? IIRC, the bolts have to be torqued in steps, and I know you have to follow the pattern. Overheating can also spell the end of a headgasket pretty quickly on a motor with a cast iron block and an aluminum head.
If you're getting the classic symptoms of a blown headgasket, like bubbles in the radiator or "chocolate milkshake" in the oil pan, it's certainly time for a headgasket. That being said, I have my doubts. These engines have a very stupid thermostat location that causes any bubbles in the system to congregate at the thermostat and cause very erratic cooling system behavior, like rapid swinging of the gauge, chronic overheating, etc. I would imagine any failure of the headgasket that would be enough to cause noticeable coolant loss would be dumping tons of air into the system from either compression or vacuum on cooldown, so you would be seeing those symptoms as well. Major headgasket leaks can lower compression enough to make the vehicle undriveable, and you would be lucky to get it to rev very much away from idle. I would do a compression test and an exhaust gas test on the coolant before I singled out the headgasket. It certainly could still be that, but I know I would not be a happy camper if I spent $150 on parts for a headgasket and spent a weekend replacing it only to have the car run not much better than before.
How did you get the data on the fuel ratio?

 

[B10alia]

I would say the headgasket hinges on the compression test. You could have a coolant leak somewhere else, like the heated manifold, that's spraying coolant onto something hot and boiling it, giving you the smell. I would expect serious idling and driving problems with a badly blown headgasket. Obviously, I haven't seen the car and how it performs, so there might be something that fingers the headgasket that I haven't seen. I know that if I were you, I would want to make sure it was the headgasket before I committed to replacing it. Like I said, a compression test will be the determining factor, but I would definitely do that before anything else.

 

[B10alia]

Weren't the original "bad" headgaskets replaced by an MLS one? I.e. isn't the current OEM replacement a MLS design?

The reason I'm still a bit skeptical about the headgasket is that the symptoms and what's expected don't match. If the car "burns coolant" as the OP stated, the headgasket would have to be leaking fairly severely. A very small leak (like the one I had on my 2.5) is all it takes to freak the thermostat out and cause issues, but the OP never mentioned any problem with the cooling system behaving erratically. Further, a very small leak would not cause a noticeable loss in coolant. I never saw a drop lost from the system, and I had a leak for months. The change is there, yes, but it's so small that it's imperceptible. As Bob L. noted, it could be a leak somewhere else in the system, giving you the coolant smell and consumption without the other headgasket symptoms. I think that most of the problem here was a bad ignition system and vac leaks. Again, a compression test is necessary.

 

[B10alia]

You can also get generic CRS bottles. The shape is just to get it to fit on the radiator; you can put the bottle anywhere, so long as its overflow inlet is slightly below the radiator cap (at least, that's how I would set it up, although I'm not sure it would matter when the cap opens under pressure. Check your local junkyard if you want OEM, it is one of those weirdo parts that nobody really ever needs

 

[B10alia]

Get the Stant Superstat. It already has a jiggle valve, I've had good results.
http://www.rockauto.com/catalog/moreinfo.php?pk=886492&cc=1098514
I believe the OEM spec is 195, but I could be wrong. You can see the jiggle valve in that picture just to left edge of the main poppet, it's that little brass "finger". No drilling necessary.