Is it against the mind of the Church when a choir alone sings a hymn at the offertory and preparation of the gifts or are the people supposed to sing along as well?
No, it is not against the mind of the Church!
It is entirely proper for people actively to listen to good sacred music during Holy Mass.
The claim that everyone must sing everything, that if they listen they are being denied the opportunity to participate actively, is a canard.
That said, the music should be good, artistically speaking and in its performance, and it must be sacred. The text should be sacred and the musical idiom appropriate for the sacred liturgy.
And that having been said, consider not having hymns at all during Mass, unless the hymn is used after the Church’s actual liturgical texts have been sung.
Let’s get rid of hymns.
The Church assigns antiphons certain moments during Mass. Those are the texts that ought to be sung first and foremost. Everything else must take the back seat to the actual liturgical texts.
Down with hymns during Mass!
Form a schola cantorum!
There are other liturgical and devotional settings when hymns can be and ought to be used.
Use the Graduale Romanum for the Usus Recentior, the Ordinary Form, the Novus Ordo!