At this point of the research, what was necessary was to

1. globally allow WFS transactions in Geoserver for all workspaces

and then in

2. Security > Data Security , allow only ADMIN to write in the workspaces you want to preserve from external modifications

like dataspacetopreserve.*.w (write) = only ADMIN and not ROLE_ANONYMOUS

At this point of the research, what was necessary was to

1. globally allow WFS transactions in Geoserver for all workspaces

and then in

2. Security > Data Security , allow only ADMIN to write in the workspaces you want to preserve from external modifications

like dataspacetopreserve.*.w (from anonymous writing) = only ADMIN and not ROLE_ANONYMOUS