1 - The Wendigo's melee attacks have a trigger that if it kills or sacrifices the target, she gains Fast.
2 - you may not want to take a beast I guess. Myranda only synergise a with other beasts whilst in human form. If you take Myranda, the Blessed is the only Frozen Heart beast for Ice Mirrors. You can make Myranda transform into others but they won't synergise as well as the Blesed will. You might like a Cerberus or Razorspine though.
3 - yes, but it only adds one to the final damage amount. E.g. Rasputina gets the red joker on a December's Curse attack. It does 8 damage (severe 5, weak 2 +1). Same goes with the Acolyte's shots.
4 - Yes. "All models".
5 - Yes, although you cannot spam the same armour buff from the same source (e.g. Rasputina's Decemeber's Touch tactical action). See miniature rulebook page 63, Stacking, bullet point 4.