It is also worth mentioning that the presence of HSA in the growth medium produced a reduction only in some mRNA species. These results speak against an unspecific hydrolyzing effect.
It is known that HSA can bind RNA and DNA and has RNA hydrolyzing activity. In this way your experiment in par 2.2. may lead to not all RNA after incubation with such amount of albumin. That is why some of your effects may have other explanations.