Is C3 cumulative? If I have three 'Mechs with C3, do they get +4, +6?
No, if you have two or more 'Mechs in a battle with C3, they each get +2, but no more.

What is a Faction?
Faction is a keyword. As per the text appearing on these cards, you may only have one in play at a time.

Where may Terrain cards be played?
You can play Terrain cards on one of your regions: Stockpile, Scrapheap, Patrol Region, Command Post and the Under Construction Region. They apply to missions that target any card, 'Mech, or site in the region.

I have a Fog-Shrouded Moors on my Stockpile and Rocky Gorge in my Patrol region. My opponent attacks my Stockpile and I block with a 'Mech in my Patrol region. Do I get both regions' bonuses?
No, the only Terrain card that matters is the one where the target of the mission resides.

With unique cards that count as other unique cards (Kai, Champion of Solaris, Victor Steiner-Davion, and so on), can you have both versions of the card in your deck?
Yes, but you may only have one in play at a time.

I have a Puma, a Fenris with a Ristar Mechwarrior on it, a Daishi, a Training Facility, and a Warrior Caste in play. I play Natasha on the Daishi. What attack and initiative bonuses do my 'Mechs have?
The issue is whether or not Warrior Caste transfers the Training Facility's effect on Natasha to the other 'Mechs. The ruling is that Warrior Caste should say "base attack and initiative bonuses," so no, it doesn't transfer the effect. The 'Mechs have the following bonuses: Puma, +3/+3 (Natasha only); Fenris, +5/+5 (Ristar w/Training Facility + Natasha); Daishi, +4/+4 (Natasha w/Training Facility).