These are the methods I use for working with the Goetia, and I have only encountered favorable responses from the demons. Some people don’t like how the angels are incorporated due to their biases against JCI religious people (and their assumptions that the angels are anything like what those people believe). It is as you say, the angels are counterparts to these demons and work harmoniously with them.
Some also don’t like how you command the demons, as many would rather treat them more like objects of worship and adoration, placing the demons above themselves. This is to misunderstand an essential aspect of this magick. The Goetia teach you to be one who rules well over others, thus the titles of Duke and King, though in today’s world perhaps C-suite executives would be a more accurate analogy.
As for visible manifestations, I should first say that I have yet to visibly manifest a demon before me as though it were physically there in the room during a ritual. I have practiced such techniques, but my current living situation is not particularly conducive to such setups, and so I focus instead on my imagination. I have had this experience outside of a ritual setting, and they were surprisingly clear, but that has only happened a couple of times.
Now with that being said, I have achieved interesting visions with many spirits many times now. With these demons, I will often see them standing in an environment that matches the Evocation Keys for the demon, and their forms will sometimes be influenced by the Keys as well. Even before this I was able to commune with the spirits reasonably well, as visual manifestation is not required to open communication. I also tend to focus more on the tactile sensations of magick than visual effects, so this has never been an area of great concern for me.
With the dark room vs incense idea, I would guess that either can work fine and that it just comes down to preference. When EA’s talked about this he described it as mapping what he can see in his imagination over what his physical eyes are seeing, to more-or-less combine the two senses. If you’re going to take the incense route, I would be sure to use a scent that matches the Evocation Keys (or something with no scent), as otherwise it may interfere with things.
I’d also use a dark room whenever you can, to leave open the possibility for more immersive experiences. I’ll often see an entirely new landscape, as the idea with the Evocation Keys is that you are recreating the sensations of the place where the demon resides, and then breathing those sensations out of yourself and into the room around you. With enough practice it may very well be possible to experience all of these sensations quite distinctly. EA’s “Lake of Fire” experience, from how he’s described it, certainly seems similar to this, so I would not dismiss the possibility.