You should use EB on main accounts and API on scraper accounts.
If your scrapers have been scraping usernames, that info should be sent to your main account so you can do actions. But you still need to always have at least one account signed in via API or else you will get error 550.
When you have “USE ONLY THE EMBEDDED BROWSER”, you need to have another valid IG account in Social Profiles tab which doesn’t have that option checked. That account will be used as a scraper account.
You can use any other source options in your main account, you don’t have to add pre-scraped usernames under follow specific users.
When you have Use only EB to follow/unfollow, you don’t have to add another IG account, but that will mean the account will make API calls by itself which can lead to blocks. You can use any sources as well.
can you log in to the account manually on the embedded browser? (Social Profiles > Browse)