No need for all the windows and menus.
Heh. The "windows and menus" are not the problem! The problem is the movement, token selection, and macro execution. Those are the things that require enough of MapTool to be present that it's not practical to create a separate tool.
Of course, if someone wants to tackle it and create some kind of "viewer" that doesn't require all of MT to work, I'd be keen on hearing about it.
2) Using macros, is it possible to pull values from a REST service?
No. There is no access to the local filesystem or network resources from inside MT. The closest you can get is an HTML anchor element that triggers a separate browser window to open. (We don't want MT to become a potential DoS platform.)