I provided a response to a similar question a few months ago, and as it addresses most of your concerns, I shall link it [here] (https://www.reddit.com/r/AskHistorians/comments/30h8qg/at_the_decline_of_the_western_roman_empire_did/cpsga1n). Scroll down to the edit to see the relevant section. Please feel free to ask any further questions!