[cleveland]

I'm currently playing LA Pan in MP, though it's a CBM game, so LA Pan is really a completely different nation: in CBM, Carrion Woods is level zero, so I put it up as soon as I could and relied on the manikin/undead animal freespawn to expand.

But, that makes me uniquely qualified to comment on a Pangaean undead strategy, which I've been using now for ~40 turns. I don't want to tip my hand (DonCorazon stop reading now!) but I can give you some thoughts.

First, to answer a few of your questions:
"Are there any other kinds of 'better' manikins available through reanimation that I didn't get so far?"
Sounds like you've seen them all, though it's worth mentioning you'll occasionally get Mangradoras, which have 2 sleep vines + claw, 25hp, respectable att & def (9), and 13MR.

I seem to remember people mentioning using an ogre manikin or something like it as a SC chassis?
LA Pan has an undead "Ettin Mandragora" hero, so you'll only get it through the "Suddenly this guy appeared at your gates...!" event. It's an excellent chassis, with 3 hand slots, 2 head slots, and an L1 priest (self bless). Vine Shield + Boots of Behemoth + Regrowth and you've got an early game quasi-SC.

Did I miss any other means of acquiring manikins?
You can cast the Const3 spell Construct Manikin, though I don't recommend it.

Now the big question:
How useful do you rate them?
They frankly aren't very useful on their own, BUT are very useful to LA Pangaea.

Why? (implied)
1) Manikins (and here I'm referring to all the undead stuff Pan gets) are completely different from everything else in Pan's arsenal. To counter them effectively, your opponent will need to field banish-spam priests, which diverts funds/attention from more cost-effective counters to your Minotaurs, Centaur Warriors, Dryad Hoplites, and Harpies. If he decides your manikins aren't a threat...

2) Massed manikins are surprisingly effective. The sleep vines attack, when it hits, is devastating to fatigue; it only takes ~3 hits to knock an opponent unconscious, after which death is certain. This is an excellent counter to small groups of elite troops, and to lone SCs.

3) Manikins have surprisingly high magic resistance. They're still vulnerable to banish-spam, obviously, but L1 priests will only connect ~30% of the time against the standard manikin (MR=12); some of the undead animals are lower, while some are higher.

4) Pan gets several undead-only national spells. Most notably Regrowth, which gives undead the ability to regenerate. Not terribly useful for a manikin, but very nice on an undead Elephant, very very nice on a Wraith Lord, and very very very nice on a Tartarian. Puppet Mastery is also good, particularly when faced by those pesky archers or when storming a castle.

5) They're 100% poison resistant. Pan is notably lacking in battlefield damage spells for the LA. Blade wind is an option against, say, Mictlan, but most troops will be wearing too much armor for BW to be of use without Destruction. But every one of your Pans can cast Poison Cloud.

6) Pan is the Buff Master. What it lacks in direct damage spells, it more than makes up for in troop buffs. First Wooden Warriors, then Mass Protection (or Marble Warriors), then Army of Lead (or Gold). Throw on some Weapons of Sharpness & Strength of Giants, and all of a sudden the mighty Tartarians of Tien Chi are crying out to the rest of the world for help.against your shambling horde of rotting orange peels. Maybe even summon a Lamia Queen or 2 with a Panic Apostate and drop some Darkness for good measure.

7) As Pan, you'll be awash with nature gems. You'll be trying to think of ways to use them, least you feel like a total dope for having 1000 N just sitting there. Carrion Ladies are your best reanimaters/cost (2 manikins/turn @ 16N gem cost). It may not seem like much, but you can easily have 10 by turn 30...by turn 100, those 10 will have generated over 1400 free, need-not-eat, upkeep-free troops.

Your 2 best reanimations are are the undead elephants & centaurs. The elephants are just like normal elephants, except they don't rout (with morale 50) & can't be soul-slayed (mindless). The centaurs are armed with a vine bow that entangles enemies, which can be surprisingly effective against small groups of elite troops.

Got to go to work, maybe I'll add more later.