If a Jakara reflects a shot from a lasgun does she use her base BS to hit or are all shooting modifiers used too?
Yes, a reflected shot still has all range, cover, movement, etc. shooting modifiers applied to it using the base profile of the weapon that fired it. As an example, if a Lasgun shot is reflected at a range of 8″ and the firer (now the target) is in hard cover then a '-1' (+1 range, -2 cover) would be applied to the Jakara's BS.