From a legal perspective, nothing can ever "void" the warranty on a product per se. However, this seems to be a commonly stated issue for many products. For example, car dealerships often tell customers that if they don't perform the standard service work their warranty will be voided. Absolutely untrue. The warranty is a legal contract and can not be voided. What is true, is that if damage is caused by user negligence the manufacturer can refuse to cover said damage. So, continuing the car example, if you don't change your oil and the engine seizes because the oil has turned to equal parts syrup and sludge, they can refuse to cover the repair.
Correct. And more to the point, if the door falls off or the transmission dies, they still have to cover those repairs.
As for Canon threatening legal action only for firmware hacks on a 1-series but not on other bodies that seems strange. I seriously doubt that's their official position. If ML is not supporting 1-series bodies it is far more likely that they just didn't want to put the work into writing code for 1-series bodies when for video the 5D2/3 is the much bigger market.
There's probably a fairly trivial way to modify the older 1-series cameras to do 4K video, but the extra heat would cause a high device failure rate because of the lack of a larger heat sink found in the later versions. So I could see that particular hack leading to legal action, if only because Canon's only choices would be to refuse warranty service or eat the significant repair costs.