There is some misunderstanding . Sorry .
So please let me ask , who is the addressee for the questions in post #8 ? That is not quite clear to me now .
To nail down the culprit for your described issue , in the moment I can only see three individual areas which are :
aaa) mechanical malfunction of camera and/or film back . To exclude the back , it would of course be nice to have an other back .
If you don't have a second back , you might be able to borrow one from a friend .
If this is not possible , you could try the following . Attach the loaded back to the camera . Pull the dark slide and then use a black insulating tape and tape all around the back
in the area where the back attaches to the camera body . Additional , you could hang a cap over the back , just to make sure that no light comes to the back . Your first image
then might show a light seal issue , but all others should not . If your issue is still seen , then of course it is unlikely that you have a light seal problem .
bbb) Check your film loading process to the spool and make sure the film is properly loaded . If the film by mistake contacts itself in the spiral , issues like yours could show .
And of course make sure no light can destroy your loading .
Check and make sure , that developer and fixer are fresh and clean .
ccc) Check the film for proper rollup on the spool and that the film has not been exposed to strong light before you load the film to the back .
Hope , this helps a bit .