The bonus chest in between echoes (not the echo specific reward crystal) drop quantity is dependent on the number of enemies you kill in the echo. The bonus stability bar fills up the same way but is capped. So they are related but not dependent on each other.
It is generally assumed that most characters will have capped resistances and it is balanced as such. However, max res is not as important as it is in games like D2/PoE because in LE, enemies also get 75% all res penetration, applied after cap. This means that it's actually balanced around everyone having 0 res (after the pen is applied). This means that not having capped res is still impactful but nowhere near as impactful.
There is no ETA on multiplayer. We have to get it right and we don't want to put ourselves in the position where we have to announce a delay.