a) On Endless Mode - I bet I could just turn off the victory conditions and the game wouldn't break it would just go on. I'll give it a shot and see if it works, but with some of the Old Ones you'd doubtless reduce the world to waste. I can't spend any time trying to balance this, we're strapped for time as it is, but if this works I'll make the Victory Conditions toggle in the editor. Then you could adjust hero spawn rate, restrict the Old Ones, and build one yourself. If it becomes a truly popular option we may make an official Scenario that has a similar idea.
b) The Game End has two meanings for us - one is the climactic struggle by the Alliance and Heroes vs the Old One and his allies and the other are the actual conditions that end the game. The main way the game ends is Destruction, which is the amount of POIs in the game that are either under your control or that have been reduced to anarchy or ruin. This means that you can either occupy the world with your hordes, create enough random destruction that the world collapses, or more likely a combination between the two. Some Old Ones have specific victory conditions, such as one being able to negate all life if he completes a ritual once he has awoken, and another whose victory is tied to the Chosen One.
The game end is generally less interesting than the climactic elements that bring you to it, which is how we designed the AI. As things get worse and worse the Alliance AI (whichever nations have decided that they must band together to stop the Old One) will grow more desperate and willing to sacrifice itself in order to stop your rise. You will see massive multi-national armies marching towards your hordes, you will have to manage multiple High Level Campaigns by Heroes trying to find powerful artifacts or perform rituals to impede you, and your agents will be brutally hunted down and killed.
The key to the world is that the nations, if unified and uncaring about their own safety, would always be able to destroy you. You must keep them on the defensive, concerned about their own needs, and not willing to join the Alliance. Alternatively, tactically weakening nations that you know will join will make the Alliance weaker in the end.