The right play is going to depend on the matchup (if they're a very fast deck it could be right to block), your hand (if your next play is a Ghoulraiser and your creature is instead a Walking Corpse, it is excellent to block), and your tribe along with your deck
You rarely recover from many of these destruction moves, as only one card, the aforementioned Song of Inspiration, allows you to bring creatures back from the graveyard
Challenger gives a character additional strength
Take your time getting to used to playing 60-card Casual, as you have plenty of time lately to get to grips with more niche formats
I think the "both is good" argument for Valki and Tibalt is that they'll be good together but there's no added value
Ruric Thar, the Unbowed absolutely chunks anyone foolish enough to cast a non-creature spell (including you, but thats the Gruul way)