~ in general those who are demonic or connected to death will have an easier time of things within the demon realm. Those connected to life/affirming energy will find it harder. Most people fall in the middle.
~ So yes in his more powerful forms he is likely to be hit harder by the energy drains
~ they would be able to sense strengths when close enough but not location
~ demon flesh is toxic to all life forms
~ Someone did say that but I don't know doom myself so it's not actually actively based on it! All similarities are coincidental!
~ They are pretty tough themselves as well as being guarded. They don't have magical resistance, they are just made of a very tough rock.
~ the portals are scattered with the closest ones in the city themselves and others a good few hours away. The furthest away are a good days hike from the city. Inside the demon realm he will find the more he flies the more drained he will become

~ I know Zetta is looking to try and take over the demon realm.. It won't exactly be successful but if you needed a distraction for him then going to try and beat their way to the top might be an angle if you wanted to plot with that player!

Otherwise he could certainly go in to try and find more powerful demons and have his own demon fighting party.