1) The game is available in its entirety on the elzra store / website - https://shop.elzra.com/collections/all
2) Even though it may be labeled "KS-Exclusive" Elzra sells all of those goodies on their store linked above... at least for now. I personally love it all and feel the game wouldn't be the same without it. That being said, I am a sucker for variety and a completionist so take that for what you will.
3) Yes, as mentioned above.
4) FOMO? Perhaps not because it's all currently available. I will be forward and say that I love the game so much that I have serious fear of losing or breaking a piece. Elzra has had some supply / production issues making the game a challenge to get a hold of. I am heavily considering purchasing another set as a backup while its available for this reason.
5) I love how easy it is to play and teach, while also having a fair amount of strategical depth. I also find that the level of fun and excitement is very high throughout the game. The art and style of the game yields itself to a fun experience rather than simply a competition.
Aron, I recall there being mega bundles on the web store in the past with everything catacombs and catacombs & castles. Is there a plan for that in the near future? As I mentioned, I'm thinking about picking up another complete set.