Well, I’ll preface this with the obvious comment that there are several different ways to play any nation, and there are several opinions on most of these questions, so these are just mine.
1) I REALLY like to have quite a high dominion score when I play Mictlan for two and a half reasons. One, your non-sacred national troops are all complete crap. I usually don’t recruit ANY non-sacred troops (other than occasional arrow-catchers) and instead save up as fast as possible for more castles to buy jaguar warriors. The second, as you guessed, is dominion push. With a low dominon score early on you’re at the mercy of an aggressive dominion push by somebody else, while with a high dominion score you have to option of very rapidly pushing your dominion in an unexpected direction. 3 or 4 priests of the sun wielding jade daggers can absolutely trample over anybody who is not expecting it’s dominion if you’ve got a high dominion score. I don’t remember the exact formula, but 4 priests of the sun using jade daggers will sacrifice 20 blood slaves per turn, which is I think something like having 20 temples per turn….which you can deploy very rapidly and unexpectedly if you happen to see somebody with a weak dominion. (that’s the one and a half reasons.. protect yourself from dominion death and have the chance to dominion kill somebody else potentially)
2) Sloth is a no-brainer. Death & drain, not nearly so much. Almost all your good mages are old, so picking a death scale will really impact their lifespan until you can research to construction-6 and get the boots of youth (or the more expensive blood ritual that makes them younger). Death also eats into your long term blood hunting prospects. Drain I hate because I use the heck out of the cheapo Mictlan priests. They’re cheap as hell, and sacred, but have only a research of 3 (working from memory here, maybe it’s 4). If you want to use them as your primary researchers (because the more expensive ones are all old) then taking a drain scale impacts your research by a whopping 33%. It cuts the other way too though, taking just one point in magic really helps out. Now, what is more of a no-brainer is taking turmoil/luck. This is because you’re going to be having a decent number of your provinces used for blood hunting – thus not generating gold income. So turmoil will hurt you less, but luck will help you just as much and get the benefit of it’s synergy with turmoil.
3) Yes, you have a 0 chance of getting heroes at misfortune-3. I think it’s a 3% per turn chance +- 1 per luck scale. The Mictlan heroes aren’t great, but they do give you access to new magic paths you otherwise can’t easily get. The death hero lets you forge skull mentors, cast dark knowledge, summon vampires and generally start up the death machine. If you can get the air guy up to air-2 he can haruespex for awhile and build up enough gems to empower him up and start forging robes of the magi (expensive, but not a lot of other uses for air gems for Mictlan)
5) Earth magic is a pain, but if you can get it (either by a non-national mage, or through your pretender) you can easily start cranking out blood stones and create quite an earth powerhouse (make sure you make a dwarven hammer to). I seldom put earth on my pretender since it doesn’t really help the sacreds, but I do keep an eye out for any earth mages I can grab (keep hellbind heart and succubi in mind, you might be able to snatch an enemy earth mage). I can’t think of any good summons that will give you earth if you don’t already have it.
No comment on the other questions. I will also point out that as popular as an uber bless strategy is with Mictlan I’ve had some pretty good luck with more conservative blessings combined with good scales. Something like single bless, Turmoil-3, Sloth-3, Magic-1, Growth-3, Luck-3. Even single blessed Mictlan sacreds are pretty good because they’re cheap enough to throw many of them around, and the nice scales let you more rapidly get to Mictlan’s middle/late powerful blood summons.