A) Can't really avoid the Lava Layer when trying to get to hell unless you're lucky enough to get a Demon ConchYou can easily just avoid the lava layer though. Palladium and Orichalcum are both definitely strong enough to kill the mechs.
B) In Normal and Expert Mode, you don't need titanium. However, people who are bad like me need it in Master Mode. And of course, new players would probably prefer it over the weaker ores. Not every Terrarian has multiple thousands of hours under their belt and enough expirence to beat the Mech Bosses with Mythril or Oricalcum.