1. Can't say for a Hummer, but they did very little for a Vette. The body spacer does help very minimally in hot weather to help the air flow cool, but not worth the price, in a Vette.
2. GM has to balance costs, emissions, and mileage and come to a happy medium. If the spacer costs to not cause a mileage increase or emission decrease, it is not worth it. Believe me, everything is tried. If GM can scrape a mile per gallon more out of a vehicle, they will do their best.
3. yes and no. Really depends on the problem. Replace and item that affects any sensor, and yes the warranty for that part, or for that condition will be voided. If the engine blows up, chances are it will still be under warranty. It all depends on the modification and what it affects. (Add a supercharger and I guarantee the engine will be out of warranty
.)
Just remember, that the Dealer cannot void a warranty, they don't issue the warranty. They act as a representative of GM. They can explain the problem to GM, but only GM can refuse warranty work...unless the dealer can consider it a hazard. (Example, lowered Vettes were hard to work on, and some dealers refused to attempt to drive them on a hoist for fear of body damage.)