Regarding tactics, you're basically identifying the same thing as me. The testudo cannot effectively attack on its own - it must do something else when it gets there, or have someone do it for them. Also, the light troops supporting the testudo's prevent the enemy from surround the formation, pulling shields out of the romans hands, and plunging sharp pointy things into their suprised faces.
As for the shield question, get something around 5-10 lbs in weight. Now keep lifting it to shoulder height. Soon gets tiring doesn't it? Now lift the weight above your shoulder and keep it there. You can support that weight for some time.