Whether the guardian/ the trustee/ the agent is liable for tax dues etc., in case of a business of minor or incapacitated person

Yes. The Guardian, or the Trustee, or the Agent as the case may be who is conducting the business for the benefit of minor or incapacitated person is liable for payment of tax dues.

(ICAI FAQ PUBLICATIONS 06-09-2017 Liability of guardians, trustees etc.: FAQ NO. 13)