1. Melodies have to be memorized. No, you only have to memorize the questions and answer them. Answers can be simple repeats with different endings or more developed commentaries (just like in speeches).
2. Rhythms must be very elaborate. No, you only have to use the rhythms which fit the meter you have chosen. Pay attention - the moment you feel the rhythms beginning to be boring, add some faster notes in one voice. This will add more interest.
3. You have to be fluent in all 3, 4, and 5 note chords and their inversions, modulations, many different keys and modes. It's nice if you are, but if you only know one 3 note chord and can transpose it in a few keys, it's a good start.
4. It's very difficult to change dynamics with swell pedal. Since you are the one who is creating the music, in places where you need swell pedal changes, you can play with your left foot only or even without any pedal part.
5. It's very difficult to change registration by hand. You can make the stop changes as complex or as simple as you want. You can set the registration in advance and change only manuals for the entire piece, you can play with one hand and change the stops with the other, or you can make a full stop to facilitate registration changes. It's entirely up to you.
6. Texture has to be very elaborate to keep listener's interest. No, some of the nicest improvisations can be done using a simple one part texture. If you are worried about the polyphonic texture, remember that the easiest way to do dialogues and imitations between the voices is when one part is moving and another is stationary. Just like in a conversation between two or more people.
7. Form has to be very advanced with lots of new ideas to be musically interesting. No, the more complex form you have chosen, the more challenges you will face. But the form could be very simple - idea A, idea B, and recapitulation of idea A (with or without some changes). Simple is good in this case.
It's important to recognize these myths for what they are - simply untrue common believes and excuses to not even try. By demystifying them, you can set aside your fear and begin your improvisational adventures. The growth begins the moment you sit down on the organ bench.