1) nononono, dont use ruby xchange for money.

I currently farm 7-1 hard everyday now reach 800k + . Hot time, exchange honor point with key and keep farming there.
got a 6* knox, knox ability can get 0 dmg from these magic monster in 7-1, and was running as tank...i not sure about other people.

2)  didnt get perfect clear on hard. assume need high lvl weapon. got all 6* also cannot clear.

3) 6* hero :
a) from halloween
b) from fusion (do it for tutorial guide thing)
c) reward of finishing the tutorial guide get 6* selector
d) save the 4* fuse 5*, then use element up rank up or fusion to 6* (take longest time, cause need to farm fodder..3* fodders so hard to get)

gem best used for:
1) daily dungeon - get the element you want, for specific char upgrade
2) costume - stays forever, for aesthetic look only, not much gameplay change, get the one you like and love.
3) gacha - this one going to get you WTF frustrated all the time.