Seasons will be a feature we will implement later this year. We are in development for them right now and let me say they will be a lot of fun Every player can participate in them (level 5 and a free of scrolls for participation) and you will be able to unlock cool stuf just by playing certain expansions. As soon as we have something to show we will let you know.
Yes you can buy the seafrer and CaK expansion in the bundle bundle without having the basegame. It´s a great combo because you can play the seafarers scenarios (my favorite) with the cak ruleset (my favourite)
At the moment you can´t see how many players are only. the most popular game ist the basic catan game (first island) but i would suggest to look out for a guild and start playing with them so you will have buddys to play with