Your analysis is NOT correct.
JMS provide you the correct path as you have used {site_rel_dir} as folder.
Check the yellow light when defining the slave site and you will see that {site_rel_dir} correspond to multisites/{site_id}
Therefore the path
<host>/multisites/<slave>/ .... is correct.
If you want another path, up to you to specify another value when you create the slave site.
Remark:
Once the directories are created, they are no more modified by JMS even when you change the parameters in JMS.
Once the slave and its DB is created, those image and media folders are saved in the Global Configuration.
So your error is to want a specific image and media folder and after expect to have it in the master or slave site root directory.
The following sample assume that you have deployed the slave site into a specific directory and that Symbolic Link is allowed.
If you want to have the image and media in the master website, you just have to let the fields empty. This will create a Symbolic Link on your master.
If you want to replicate the master image (
<master>/images) into a specific slave image (
<slave>/images), you just have to specify the images and images/stories (without the {site_rel_dir})