Hi,
firstly, s2member custom_fields are being stored in wp_options table not in wp user meta table.
secondly, plugin stores all data in correct format whether as string or in serialized form when saving any array. only array data is saved in serialized form.
To enable s2member fields with registration, I will add this feature today in my pro version of plugin JSON API User Plus, and let you know.
Lastly the correct way of calling url, when it will be available:
https://www.chkun.com/api/user/update_options/?cookie=". $ arg1. "& meta_key = s2member_fields & country=Germany&city=Berlin&mobile_phone=0123456789
All this data wll be saved in serialized form and you will also get the data from other end point:
https://www.chkun.com/api/user/get_options/?cookie=". $ arg1. "& meta_key = s2member_fields
I will let you know when available.
Thanks