Can't reach competition depth in the squat. If you can reach proper depth with little or no weight then it's a strength- and/or technical issue and not a mobility issue. In that case you need to check your ego at the door and practice with weights where you don’t cut depth. If you can't reach proper depth even with no weight I suggest practicing squats daily, even on non-training days. Simply pick a time of the day when you squat down and try to get comfortable in the position. Holding a small weight in your hands can be helpful. Your squatting abilities will improve quickly. Of course keep practicing the lower body stretches in this guide.
Want to improve the arch in the bench press. First, this is only part mobility issue, it's often also a technical issue. Practice a proper setup every set, including warm ups. Proper setups are outside the scoop of this book. As for the mobility issue, practice the thoracic extension in particular but shoulder dislocates will be helpful too. Also make sure to stretch your pecs after training.
Can't reach the bar with a flat back in the deadlift. Try lowering yourself to the height where the bar would be, without using any weight and while keeping a flat back. If you can, it's a technical issue. If you can't, it might be a mobility issue. A quick fix is to try and widen your stance, you might be able to deadlift with a flat back immediately. You can also start doing deadlifts off blocks. Gradually lower the blocks (over the course of several sessions) as your mobility improves. Keep practicing the good morning-, pretzel and hip flexor stretches.
Can’t get a wide stance in sumo deadlift. More often than not I think people lower themselves too much in the sumo deadlift so first consider if you aren’t actually sitting with your butt on the ground. If that’s not the case, practice sumo’s off blocks and gradually lower them over the course of several sessions. Second, try the horse stance. This old Shaolin favorite is useful for many people, including sumo stance powerlifters. Essentially it’s a static squat with no weight. Differently from the squat, point your toes straight forward and even push your hips forward slightly, in other words don’t sit back as much as you would in a normal squat. It’s important that you stay upright. Hold the bottom position, it doesn’t have to be below parallel. Gradually widen the stance over the course of sets and sessions.