Computer screen can't be seen by the rider during the race so under the saddle, under the stem, screen taped off are all legal way of keeping your computer on the bike the race. Just make sure it is properly attached so it won't come flying out of a corner.
Helmets and sprinting: Short-tail TT helmet like Kask Bambino, Lazer Wasp Air (or the soon to be available Lazer track air), Poc Cerebel, Bontrager Aelous, Catlike Rapid and Spec S-works TT are good option that won't took much of an aero drag reduction when you turn your head to spot your rival during a match sprint or in a keirin heat but will still be faster than normal round helmet. Lazer, Kask and Poc short-tail TT helmet have been accepted in the past at world cup level for mass-start race. Laura Trott always wear her Lazer Wasp air in every mass-start race.
The idea behind the popularity of helmets of low-profile round helmet like the Casco Warp, Giro Air attack etc is that the constant looking around in sprinting would mess the aerodynamics of TT helmet. When wind tunnel access got easier, that idea was quickly debunked. The Giro Air Attack is less aero than most normal vented road helmet at any degree of yaw.