Can fast sex shaken up or harm the developing fetus...cause any problems? Brain, etc? Sexual positions, speed, intensity are factors?

NO. Having sex will not affect the baby. Use common sense of course and i would not have severe rough sex as repetitive trauma to the cervical area, may irritate your internal vaginal structures and thus cause possible problem with your pregnancy, BUT, in general, having sex will not affect the growing baby. As you get farther along, late in the 3rd Trimester, discuss with your OB/GYN. Best wishes.
Unlikely. Usually regular sexual activity does not hurt the fetus. Enjoy your self, take prenatal vitamins, avoid smoking or drugs and eating healthy meals and minimize stress.