Shouldn't it be 'ná déan é sin'?
That shouldn't be marked wrong. Both are correct though. One just depends more on context.
I always though the root was used for commands and the imperitive was for more lets do or should do
What you're thinking of is referred to as the optative mood. "Let's go to the cinema" would be in the optative mood, whereas "go to the cinema" would be in the imperative mood.
Is there another conjugation for optative?
“Let us …” is hortative; “May we …” is optative.
Irish expresses the hortative with the same form as the past subjunctive, e.g. Téimis chun na pictiúrlainne!