How can I display/list the devices that are currently signed using my details (BTid/EMail/Password)? With other services for example Google/Yahoo and many Apps (Netflix, Amazon) I can list the devices and decide whether they should remain logged on or disconnect them. Why can BT not provide this service?
Probably because they're not required to keep a database of your online activity. If you use a search engine to find the answer to your primary question, you'll find lots of websites giving you advise
All of the above. How and where you use your email address for login credentials is entirely down to you. Neither BT or any other email provider have any legal requirement to keep a record of them. They have a legal requirement to keep a record of websites that you visit for a certain amount of time but they only have to make those available at the request of a court of law.
Google do keep a record of any signins linked to a Gmail account but it's their choice to do so because it keeps you within their ecosystem
BT won't even be aware. If a text string that just happens to be in the form of a BT email address and password is used to login to a third party site BT will have no knowledge of it.