You won't sacrifice a creature
Once a creature that has reach because of Sphere Grid's last ability has blocked a creature, removing all +1/+1 counters from that creature or causing Sphere Grid to leave the battlefield won't cause that creature to stop blocking
Ranger-Captain of Eos Ranger-Captain of Eos looks excellent to me
There are plenty of existing archetypes which naturally build a high storm count just by chaining together cheap permanents, be those elves, equipment, or expendable bauble style artifacts
Evolution Witness Eternal Witness isnt on this list because its not an elf, but can I interest you in Evolution Witness
This ability makes Angus Mackenzie the most optimal commander choice to spearhead a deck of the polarizing "Turbo-Fog" deck archetype, as it can be used to reliably prevent combat damage during each turn rotation and stall games