Back to your first question...unless the Realtor has authority to act as the Owner's agent (in the legal sense of "agency", not as a real estate "agent"), then you need to have the owner sign your contract. As others have noted, a retainer would be in order for such work.
Keep in mind that you might not have any lien rights. Our lien rights as professionals is not always the same as the typical "mechanic's lien". Check your state law on the subject. In some cases, for a professional to have lien rights, there must be improvement of the property.
You might want to consider using a standard retainer agreement for such work. I use one for most litigation or investigative work.
Are you doing a property condition assessment as in ASTM E2018 or are you doing a "home inspection"?