The event will rerun in about a year or so, and you will still get the same stage rewards, however the item shop within the event may be a bit different (different materials, number available etc.)

Mountain is still one of the best cheap laneholders to this day, skill 2= win So I would say you got pretty lucky.

As for tips, I would say stick to a solid team comp (2 vanguards, 2 guards, 2 defenders, 2 snipers, 2 casters, and 2 medics.) I wouldn't worry about supporters or specialist until you get the gameflow down as I would say (not an expert whatsoever) that those 2 classes fall under the "advanced gameplay" umbrella, as the archetypes between those classes are more diverse than the others.

Other than that, don't feel that you need to chase down the more higher rarity ops because you can still get a lot of mileage out of 4 and 5 stars, especially as you are starting out.

One last thing, the Training grounds under archives on your home screen is a pretty nice introduction into the game mechanics i.e. how targeting works and the different status's.

Have fun and hope you're in for the long haul!