It's OK doing the shoulder exercises to failure during the offseason. I never did them that way, but some of my teammates did. No problems at all. But give yourself a solid day or two rest in between shoulder workouts to failure, as it is, of course, more taxing on your arms than doing a more traditional set/rep workout.
I'm not sure I'd do the med balls to failure. Six to eight reps is good enough. Most people can't keep their form beyond eight reps. If you can, you're not using heavy enough med ball.
Keep working hard! It sounds like you're doing a great job.