I ran into a similar situation recently, and have one trick you may want to try before pulling the manifold. I am in the process of replacing fuel injectors on an old Nissan Maxima and saw the same oily, gritty residue inside the manifold, which I attributed to the flow of
oil vapors through the PCV, but at 160k miles this should not have been a surprise. I knew I wouldn’t be able to get it squeaky clean without taking it off of the engine, but I did want to clean the bunghole area for each injector and any other places I could get to that appeared to have excessive buildup. I could see the back side of the intake valves through the bungholes and they actually were surprisingly clean.
To avoid having any residue I scraped off end up on the valves and subsequently into the cylinders, I took my shop vac, attached the crevice tool to the suction hose, and then stuck a piece of clear rubber hose (about a foot long and small enough to fit into the bungholes) into the crevice tool and wrapped tape around it at the crevice tool to hold it in place and close up gaps in the crevice tool opening around the hose.
After scraping inside each bunghole I would then suction out the residue either through the bunghole or the intake runner. The intake manifold on this car is made in three sections stacked on top of each other. To gain access to the injectors you have to remove the top two sections. This actually opens up each intake runner and helps in being able to gain access to other areas of the manifold, which may be more difficult in the one piece manifold of your engine. But the suction of the shop vac did remove residue that I was scraping off that ended up on the backside of the valves.
In your case, you may not be pulling the injectors, but maybe you can use the suction trick to remove what residue you can get to through the top of the manifold.
I would NOT recommend this method in conjunction with use of a volatile cleaner for fear of the vapors passing through the electric motor of the vacuum and potential sparks in the motor not playing well together.