Is it permissible or impermissible to obtain a certificate of experience from an institution where one has volunteered – in a minor capacity and from home?

It is not permissible to issue an experience certificate to someone who does not master the skill, because it is a lie and forgery, and the work resulting from it is deception, according to the Prophet, peace and blessings be upon him: "Whoever deceives is not of me."

If you do not master the field for which the certificate will be granted, it is not permissible to take it. But if you do master it, then there is no objection to that. It is not a condition that you have formally worked in the issuing institution; rather, it is an attestation from someone who ascertains the person's expertise.