The difference in the user owner of the IRIS process, System Management portal is a web process and runing under system user of IRIS irisusr, and your terminal process is under your user.

well, in this case you may use method __setitem__ directly

do ws."__setitem__"("A1", "blah")

Should be quite simple, in some cases even without any configuration. Any ClassMethod has a shortcut to Debug it

Some more settings for different cases, you can find here

VSCode has such feature, from Menu Go -> Go to file, (cmd+P/ctrl+P)

Yeah, that's really, so. Redis has that feature, so you can set a time to live for the value, and it will disappear after that. In IRIS The only way to have something that will be deleted automatically is, Process-Private Globals, but their time to live is tied to the process, and you would still need to clean up if your process is long living.

Any reasons for doing this? This is not how it is supposed to be. REST should answer with Status 401, and optionally  with methods available. And web application when gets 401, it knows that it has to authorize the user, and shows its own page or initiates SSO, depending on the task.

Anyway, if really do this way, It looks like %CSP.REST extends %CSP.Login, and it has Method Login

Called for a REST page in the event of a login being required

Did not test it, but I would expect it will do what requested

You should ask WRC, for the help if you have support 

Atelier API is just for IDE, it's not supposed to call any custom call.

There is a way, that you could call SQL Proc, via SQL Query. But it is not what I would recommend

Api Atelier was added with 2016.2, and it's the oldest version where it's available, you can't migrate it to an older version. You may ask InterSystems or someone else for help in the upgrading process. 

Not, it's for Windows only. 

Instead of Studio, you can use VSCode, it's available for any platform.