Q: how do I find every reply a certain account received, not just their own posts?
Use to:username for replies sent to them, from:username for what they wrote, and @username to catch any mention at all. You can stack several accounts too. The form I use lets you paste profile URLs instead of handles, and when you add more than one it automatically wraps them in an OR group, so from:alice OR from:bob gets built correctly. Add min_replies:10 if you only want the conversations that went somewhere. It opens the search on X directly, free, with no login. Want to test it? Try a tool here