Yes, we use Component very heavily. No, we don’t use its reset function.
No, I don’t run into problems with multimethods: you can use (def multi-fn nil) to remove a multimethod definition so it can be predictably redefined (putting that above your defmulti should be enough).
No, I don’t run into problems with ordering in namespaces. I suspect that’s a combination of 8+ years of Clojure experience, being careful to check for first use of functions when I do move things around, and never typing directly into the REPL (I always eval code from a source file, so I use “Rich Comment Forms” for non-production/exploratory code in my source files).
Also, we have a stub test namespace for every single source file so when we run clj -A:test:runner we get a failure if there’s an ordering problem – and we run our automated tests a lot, locally! We have a script that generates a stub test namespace for any source file that doesn’t already have one: it requires the source namespace and refers all symbols as a baseline for “can this source file be loaded” successfully.
Perhaps another aspect of my workflow that helps with all this: whenever I change any code at all, I always eval the top-level form that it’s in – so the REPL always has the up-to-date version of my code. That’s just habit, but it’s also part of a good REPL-Driven Development workflow: make small changes and always test every change in the REPL as you go. That means that instead of making a slew of changes across multiple namespaces and hoping you got it all correct, you work from the bottom up, testing each changed file as you go.