1. Adding a sidebar is possible, and I just need to tweak the UI so it fits the narrow space.
However, the sidebar will be automatically opened by Firefox whenever the extension is installed or updated, before Firefox 62 [1]. I think this is very annoying for some users (including me), so it may be better to wait until Firefox 62 is released (2018-09-05).
This issue has also caused uBlock Origin to temporarily remove its sidebar.

2. You can change the option "Save files to" to "Download folder". Only this folder can be written quietly by extensions.

About the feature request:
What do you mean by "resumed from download page links"? Currently if a download task fails, it will show the "fail" icon, and you can double-click the task to resume.

[1] https://bugzilla.mozilla.org/show_bug.cgi?id=1460910