‘Mingling with Gods, he can plan for his brethren.’ What do we think that means?)
Mainly, I think it means Phelps didn’t really think the lyrics through all that much. You’ve got an omniscient, omnipotent, timeless Heavenly Father in charge; what’s left to plan? Planning implies a future, which is meaningless once one becomes timeless; if the entirety of time is simultaneous, future, past and present are irrelevant.
Also, planning, like economics, implies that there’s some scarcity present; if you have infinite funds, there’s no need to budget, and if you have infinite time and all other resources, there’s really no need to plan anything.