Sarlac can be useful in certain decks

mainly as a sacrifice to spawn other bigger minions as it always comes back.
Legendary drop rates are decent enough in Core Orbs, i think it's like 1 in every 4 packs will contain a legendary, shimzar is like 1 in every 5. And the new expansion per orb you get 3x3 of random cards in the set, you can't get duplicates either so 13 orbs gets you everything. It's crazily cheap too at $20 for all if you can't wait to get them with gold, and really some of the cards are batshit insane so this is the place to put your gold/money atm.
Remember to use my referral code for a free pack too!

Keeper is pretty nice, but mainly in decks with lots of higher mana cost minions ie he's great with Magmar.
Yes Abyssian starter is maybe the strongest starter deck, i also liked the Vanar one.
As for prismatics, save your dust until you understand which faction you like to play

then you can work out what you want to spend it on. Usually best to spend it on blues/purples, don't waste it on a legendary.