All the other characters really are chosen passing down the essances through time until they all gather at the Roivas mansion. But then, what was the point of Anthony and Paul Luther's Chapters? They both just died horribly because of what they find and end up influencing... nothing. They literally served no purpose whatsoever. So how exactly were they still "chosen"?
Why was Mantorok hanging around in a temple that could bind him?
The wall murals in Lindsey's chapter reveal that Pius tricked Mantorok's worshipers into working for him, so one can assume it's not that the temple was pre-build with the ability to contain an Ancient. Of course, if we are to assume it was and Mantorok purposefully rested there knowing he could be bound.... it was all according to plan so he would be bound by Pius, who would eventually be beaten by the chosen 13, who would eventually summon the Ancients to kill each other across three different multiverses and leaving Mantorok as the only known and now most powerful and unopposed Ancient. To a Great Old One, two millennium of suffering is worth the pain to reach his eventual goal.
Why red, blue, and green instead of red, blue, and yellow?
Probably because those are the three colors your eye can see. Go look at old newspapers that are in color. Yellow is made of tiny blue and green dots.
Do Mantorok, the yellow god, and (presumably) the orange god balance each other out like Chattur'gha, Ulyaoth, Xel'lotath do?
Couldn't Maximilian just have shown everyone the ancient city hiding in his basement to explain why he had to chop up the servants? Wouldn't that have been enough evidence?
"Hey I just chopped up all my servants in a fit of madness, want to go down to my basement with me?"
In addition to the above troper it's very possible that the Forbidden City is only there when it feels like being there and trying to bring more people might not have done anything, or worse Max might not have brought anybody back where they might become possessed.
It would have been enough evidence, that was his plan, but his children already suspected him of going insane so when they found he killed those servants it is likely they did not listen to him, having "proof" he was insane now.
Once you've started shooting and stabbing people because you think a monster is living inside their brain, I think it's safe to say you've officially abandoned rational thinking. And, as the first reply alluded to, anything Max says about the Evil City will simply be dismissed by others as the fevered rants of a gibbering madman.
Why did it take 48 years for Pious to murder Edward?
I think there may be an explanation for this. It's not so much that it took 48 years to kill Edward. It's that he killed Edward right before his plans finally came to fruition. Had Pious killed Edward immediately after Ehn'gha's destruction, someone else from his family might have taken up his banner. By leaving him alone, it would allow Edward to make plans to stop him. And then, by killing him right there at the end, it would mean that whatever plan Edward had for stopping him would be thwarted, since he'd be too dead to carry it out. And usually, a gruesome murder has a way of keeping nosy relatives away in the short-term. A much better question is this: why didn't he kill Alex when it became clear that she wasn't leaving and was following in his footsteps?