My form asks for someone who has known me for two years. Can the notary sign it?
Not that part. A notary confirms your identity from photo ID and that you signed in front of them. The pension office can tell you if that is enough.
Can a relative bring the life certificate in for me?
Not in your place. The person the notary certifies has to be at the desk with photo ID. Where the paying office accepts a power of attorney, the attorney comes and signs.
Is a commissioner for oaths enough, or does it have to be a notary public?
That depends on the country. The UK form lists both. The German page names a Canadian notary public only. This office holds both appointments.
Related pages:certified true copies, power of attorney and apostille.