I'm going to answer these in order of how you posted them to avoid confusion
1.) If you ingest the poison while immune to it, you would say no effect and the timer would never start, even if you lost it right after ingesting the poison.
2.) Theriac cures poisons and diseases. The term was meaning any poison effect would be neutralized, so in your example if you fed theriac to someone under black out juice, the black out juice effect would end. It would not heal wounds caused by like a 2 step venom, that's what it means when it doesn't cure the effects of the poison. (again a status like Poison pin, would be removed.)
3.) Yes, you are immune to magical effects so would not be pushed out of the unhollow, You can also walk through someone's sanctuary.
4.) poor editing on the wiki. You can double any poison weapon effects on a goblin iron weapon. (in the goblin iron weapon description)
5.) We changed fighting with a shield to count as dual wielding for the purposes of damage, so if you used balm of vigor and used a sword and shield you would still be capped at 2.
6.) Rejuvenation elixirs remove disease and not poisons, theriac does poisons and diseases (also Lycanthrophy I believe) at the same time.