How is growth rate an indicator of carbohydrate use?
Since carbohydrates are here the only carbon and energy source available, its assimilation is necessarly correlated to biomass production. The relation is most likely to be linear. Now, growth rate can be seen as the biomass production speed. So the fastest is carbohydrate use, the highest is the growth rate.
How do absorbance readings from a spectrophotometer show growth of bacteria if the absorbance readings are taken several times over the course of a day [and plotted on a graph]?
This is explained by the Beer-Lambert law. Simply put: the absorbance of an entity is proportionnal to its concentration - all else being equal. So, the more your bacterial population grows, the highest becomes its absorbance.
If you find a lower absorbance value at time $t+1$ in comparison to time $t$, yes it could mean that the bacterial cell number at time $t+1$ is lower than at time $t$. However, we know neither the protocole you have been following, nor the values you have been obtaining, so there could be other explanations, such as error of measurment.
Do the presence of substrates affect the absorbance reading in other words do carbohydrates have any turbidity?
Yes, molecules in the medium can impact your Optical Density (OD) values. However, that would only be true if these molecules absorb light at the wavelength you are using to measure absorbance (eg $600$ nm). This is why you should always blank with some filtered medium (or at least with some fresh medium).