If the answers to these questions were easy, this game wouldn't be so much fun!
It really depends on the game. What does the rest of the board look like? What is the most likely outcome if you do __________? Really kind of hard to answer any of these questions without looking at a specific game instance. Even then, there likely isn't 1 "right" answer but rather several possibilities that you have to decide between.
Looking at other game AAR's might be a good way to dissect some of these topics and see how people proceeded in those scenarios.
What do you do about your ally once you two have eliminated the odd man out (e.g. Juggernaut vs Austria, England and Germany vs France)?
Take just this one... Say you are Turkey involved in a juggernaut and Austria is dead or at least mostly dead. What do you do next? Well, what does the rest of the board look like? Is there a strong alliance that has formed in the west that the two of you now have to tackle? If so, stabbing the other maybe suicide. Is the rest of the board unstable? If so, you could stay true and continue to gobble up centers and see how things go. You could also use a well time stab to keep Russia from getting to powerful. But which is the better/right/preferred option? Really hard to say without looking at a given board. Even then there could be multiple right answers.