Uhh... no... ilevel is item level. Gearscore was the first addon that exposed item level and summed it up. The item level used to be hidden, but Blizzard put it on the tooltip after gearscore became popular.
There is literally zero difference between gearscore and item level.
- - - Updated - - -
The raid does not end with zero rewards if someone leaves the group. In M+, if someone leaves, everyone in the run gets no rewards.
Someone leaving in M+ is no different than running a raid and kicking PUGs right before the boss dies so they get no loot.