Does it have to be publicly shown, that the bad feelings have gone?
I don't think so at all. It can be entirely silent, imo... only time it needs to be otherwise is if an offender asks for forgiveness and you speak to them, "Yes I forgive you." (Then it's for both of you, imo.) The Truth and Reconciliation Commission in South Africa did make it a public process, which seemed to contribute to the redemptive feeling of it. I guess all that matters is that forgiveness be experienced not just bragged about.
Perhaps the startling thing about the Amish is the lack of anger?
Yep. I remember a recent case where there was rape of a young girl by sibling/cousin, and they were prosecuted by the "English" authorities. When he was given a hefty sentence the Amish onlookers erupted in dismay...sympathy for him. Later some commentator talked about how crucial forgiveness is to their worldview. He had confessed in their own church service and been shunned for six weeks. Once that was over, the Amish re-embraced him and it would have been a serious sin if they had not. (Meanwhile, some girls have been raped for years, and a six-week shunning does the trick. Hardly seems like justice from my perspective.)
There's a dark side to the Amish though, like with all primitive cults...the usual. The girls are often blamed (e.g., "You didn't fight hard enough") and not defended when they are raped. And apparently there's a good deal of that.
Quite a few girls have run away.
I think the Amish practice forgiveness is real and beautiful. But their rigid gender roles lead to abuse of girls, what else is new...

Hops