What happens to a site made with muse, if the webdesigner no longer has a valid subscription ? will the site and it plugins still be working. ?
Can the site be edited in a normal editing program like notepad++ ?
Can another developer import the site in his muse, and edit it, as if he has made it. ?
Nothing happens to the site! It will remain untouched and you even have (limited) editing possibilities using Inbrowser Editing.
A coding crack may edit the HTML output of the site with an editor of his choice, but nobody will recommend that.
A different developer can't "import" the published website into his Muse, because Muse definitely needs the .muse file to work with and not the expoted HTML files. If a developer has this .muse document, he may edit and republish it deliberately.