The negativity has kind of gotten out of control. However I do like what you suggested, where the sentry is immune to damage unless under attack by minions. They could also balance this out by making the sentry’s shields weaker, but only able to be damaged by minions. It would be similar in concept to other MOBAs, where you can only attack the enemy core after the Inhibitors have been destroyed.
Also, while I doubt the third lane is considered cheating, I bet it will be changed with the upcoming patch. I have a feeling (or rather a hope) that there will be some significant changes to the Incursion Map Layouts.