Cave keys are never irrelevant. Highest runes I've seen on someone so far is a level 12. Always having at least 1050 during the rebate event is recommended for the ultimate essence. Treasure sage is recommended only if you get the 30% at least but preferably the 40% discount. Otherwise at least 20% on the daily specials for advanced refines. Best event though is the TenTen treasure one though it requires ingots not coupons. Highest level maga I've seen so far is like an 8? You'll get their naturally over time so unless you have a lots to burn, you can put them into other things instead. Myoboku is pretty good for power still. Esp. since it affects all characters.