It all makes sense for the most part as long as you keep in mind he can control areas of his body being intangible or not. His entire body doesn’t have to be intangible at once, he learned to control that. The first time, face through the wall... Mirio can phase certain parts of his body while solidifying other parts. Mirio shows us this in a diagram during his explanation. Basically, he'd move his face through the wall, keeping the outside of his head permeable (his parts in the wall) while his face and body solidifies (so he can interact with the environment and also not fall). He does this in his fights when his arm passes through Chisaki’s hand but his fist punches chisaki’s face. The second instance, face from the floor.... His face is solidified obviously, so his face is holding his body from falling. He could have shot through from the ground, but stopped himself from emerging entirely and reached that position.