Miguel, I am not sure we have addressed the question: “can it be concluded that necessarily there is no pinching in the middle of the lips?”
Short answer: if you sound good and you do not feel choking or straining, you are most likely not pinching. Remember the 1% rule. Trying to play more efficiently by eliminating unnecessary tension in your body can include facial muscle tension and too much tension in the lip corners (not the same as aperture corners) is a long term commitment. Unnecessary tension can be found everywhere: such as the neck, shoulders, stomach, etc.
great video on shape and resonance:
Now longwinded stuff:
One of the ways you know if you are pinching is sound. When the “notes speak” do they sound like Greg’s examples? Greg is trying to teach us to play efficiently, which is to play each note with resonance and the least amount of necessary tension. You should have a big open sound and be able to play notes softly, loudly. Your shape matches your trumpet tubing length and it “sings”. As you ascend, you should know that c below the staff and g in staff have the same length of trumpet tubing. How is this possible? This is where your aperture corners, tongue arch, and jaw all work together. The pitch produced is a product of the body – trumpet system making the lips vibrate to make the note sing. The body part is what we are learning to use efficiently. By the way, since we are all different facial shapes, the shape I create for resonance may be slightly different than yours.
Watch this video:
CAUTION: THERE IS MORE TO PLAYING HIGHER ON A TRUMPET PLAYING VELOCITY OF AIR! You have lips that are vibrating and energizing the air according to the properties of the mouthpiece and trumpet. Many people say playing higher requires “faster air”. This leads to people muscling air through their horns by overblowing. Instead of talking about airspeeds, it is better to focus on shape!
