You suggested that Harry Amber doesn't discover that his sister Sally has fairy godparents until he became an adult, 20 years later, and that he discovers that his sister has fairies, and you suggested that he would be really angry at the revelation, as well as his wife Alyssa being friends with the Anti-Fairies Nats and Anti-Eliza in the past. I think that if Harry finds out, he'll go through five stages of grief (based on the Kübler-Ross model; even though that one's related to terminal illnesses, it could apply to fairy revelations too).
When Harry first discovers that his sister has fairies and his wife was once a friend of the Anti-Fairies, he first goes through denial, and then anger.
In the next episode where Harry appears, he goes through the bargaining and depression stages.
And in the third episode where Harry appears after the revelation, he finally goes through acceptance, accepting that the whole thing of his sister having fairies and Alyssa being former friends with the Anti-Fairies is in the past and he should enjoy the present while he can.
Yes. Except that in third day, he wants revenge on his son for his bullying of godkids as well as causing trouble for the godkids and their fairies. Also, Harry had been working for Ash after learning the existence of fairies from him. So, he became his unofficial agent.
He would not go through acceptance until Charlie Amber is reduced to nothing and was sent to summer school. However, Harry would go through acceptance if Charlie was sent to military school instead, but Harry decides not go with that. Instead, he will make sure that Charlie will not have friends with godkids but keeps his memories.
In the 2nd Dimension: What Could Have Been, after 1st dimension Charlie Amber is defeated by his good 2.D. counterpart and gets replaced by him, Harry goes through acceptance that 1st dimension Charlie has been got rid and was sent to Planet Destruksyon, never to be seen again.