Guten tag, ofenltt. I can assure you that your English is better than my German, although, as you can read, my German is impressive. Very, very impressive. I have listened to my share of Rammstein. Wunderbar.
Your knee sounds fairly fucked up. What was the condition of your meniscus in the MRI? How about the collateral and cruciate ligaments? Any word on fraying of the quad tendon or patellar ligament? Is the retinaculum or the joint capsule damaged? There are a lot of things that can be pissed off in the knee. It is unusual for tendinopathy to be accompanied by blood and fluid accumulation. I can pretty much assure you it has nothing to do with your quads being stiff and inelastic. Your knee suffered a trauma and that is the likely cause for this fluid accumulation. I strongly suspect you will not be able to stretch your way out of this.
When you stop lifting, does your knee stop hurting, or at least get better? There are times when squatting and deadlifting are not advisable. This may be one of them, at least until you get a handle on what is going on. I think you need a second opinion from another doctor.