If during the institution of acolyte prayer I didn’t respond amen due to carelessness or involving prayer, is it valid?
Of course it is. Any prayer said with devotion and intention is valid.
I notice in our group when we pray the Our Father many people don’t (or forget to) say Amen at the end because they’re so used to the way we pray it in the context of the Mass. It’s not a “deal”, let alone a big deal.
And in the Latin Rite, acolyte isn’t a “minor order,” but a ministry.