The estoppel certificate is typically signed by the party who is providing the certificate. For example, if a tenant is providing the estoppel certificate to a landlord, the tenant would sign the certificate.

yes

Typically, the management company or treasurer of an HOA or Condo association prepares the Estoppel Letter, Form, or Certificate.

Your broker and real estate attorney can help you obtain the certificate you want.

It can mean where an estoppel certificate is required of a landlord (they can are also sometimes required of the tenant), where the landlord is required to make certain representations regarding the state of the lease (neither tenant nor landlord are in default), the state of the underlying property (e.g. no encumbrances, or no default on mortgage, or no condemnation proceedings) or other representations at the request of a lender (to the landlord or the tenant) or a buyer of the property.