Diastasis rectus from pregnancy (10 months p.partum) is it worth spending time on planking and other core exercises? will they help with core at all?

Exercise may help: Abdominal and core exercises may help to strenghten the abdominal muscles and as a result may improve diastasis recti. This is not an hernia as there is no defect in the rectus fascia, hence there is no risk of bowel herniation or strangulation. If it persists despite exercises and interferes with your daily activities then you may consider surgical plication of the rectus muscles to each other.