About a hundred people saw the streak rise from the surface. Cargo doors don't do that.
The diagram you provided is a picture. In two dimensions. It is impossible to rule out a given 3-D trajectory based on a single 2-D rendition. For example a plane flying directly overhead, passing through the exact zenith, can only "decend" towards the horizon after passing the zenith, no matter which direction it flies, since you can't get any higher than straight up. This is even true if the plane is gaining altitude as it flies - it will still be flying towards the horizon, "down," after passing the zenith.
It's an elementary consequence of perspective.
If you really wanted to know what the full 3D trajectory of the streak was, you would combine multiple witness accounts, complete with bearing lines and so forth, and plot out what the entire 3D trajectory had to be, in order that each individual witness saw what they saw, from where they were.
But for some reason, neither the NTSB nor the FBI seemed interested in doing this. Why do you suppose that is?
In fact, only Goddard and Donaldson seemed to have taken any interest in this; they made such plots but the FBI and NTSB didn't. Why do you suppose that is?
"Of the 183 who observed a streak of light, 102 gave information about the origin of the streak. Six said the streak originated from the air, and 96 said that it originated from the surface. Of the 96 who said it originated from the surface, 40 said it originated from the sea and 10 said it originated from land. One hundred and twenty-eight witnesses reported an immediate end to the streak, 85 described it ending in an explosion, 32 said it ended in a fireball, and 11 said it ended in a flash."
Now if eyewitnesses are always reliable, and we should take their testimony as gospel, how come there are so many variances in just the 183 who said they saw "a streak." And what about the 550 witnesses who reported no such thing?