looking at porn is not cheating in my relationship but I know several relationships it is cheating. I would have her put parental filters on so it's harder for you to get too giving her more trust. I think she probably feels worse about the lying and subsequent divorce conversation then the porn.

Schedule a couples therapy session and just tell her the time. I feel both the BP person, and each of there family members (including your son) need individual therapy to stay healthy.
Do not blame yourself for the miscarriage there are so many reasons that can happen it's not right to blame it on you or her