For the DNA question, this is where having knowledge of molecular biology comes in. DNA is the template for mRNA, but the opposite is not true. Let's use an old-fashioned printing press as an analogy. And to make it more on-point, let's say that press is printing out detailed instructions for producing a specific widget. DNA functions as the typeset, which imprints the set of instructions (the mRNA) onto a piece of paper, then a craftsman (the ribosome) reads the instructions to make the widget (the protein). Putting a new, pre-printed set of instructions into the press can't change the typeset. It only goes in one direction. The same is true with the relationship between DNA and mRNA. mRNA can not alter DNA. This is true with every living cell known to science and is one of the fundamental concepts of modern biology, with only one exception- cells infected with a retrovirus, such as HIV or the Human T-lymphotrophic virus (HTLV), which have their own enzyme, reverse transcriptase, that can transcribe DNA from the virus' RNA. Otherwise, fearing that mRNA can alter DNA is like fearing that the sun might rise in the west, or that gravity might reverse and cause objects to fall up. It simply doesn't happen.
So, basically, I use the printing press analogy, and that seems to work with most people.
We've been over the "side effects 10 years down the road" question enough times already.