IceCatMobile is a Firefox fork. Firefox uses their own web engine (Gecko for rendering and Spidermonkey for JavaScript parsing and execution) and their own downloads manager, so it doesn’t depend on those services, 
Yes, please, and report back. Lightning Browser, for example, can’t download anything with Downloads Provider disabled and fails silently. But it seems to surf the web just fine with System Webview disabled, 
Uuuh, stop cyclic threading, please, hahaha. I said above that browsers with their own web engines and apps with their own download managers (Firefox-based ones, Chromium, Brave, etc) won’t fail.