Don't forget that the injuries weren't due to poor physical condition, it wouldn't have come back so quickly 2x. But the fact is that during an injury, training will be limited, which affects fitness. Also, the system may be wrong, a player may work like a horse but due to incorrect exercises may not be ideally prepared.
About the leg movement, that's Laine's style, one game he does it, the next he doesn't. If he played on the wing he would continue to do so even if he had the fitness of a triathlete. Sometimes I feel like only explaining the baseball bat will help here. Maybe that's why it's better if he plays with players like Jenner, Voronkov, Chinakhov, Texier... in short, when he has to move, since they play more vertical hockey.