Learn English – How to use ‘mainly’

Yes, "belong" can indicate ownership. Or perhaps I should say the opposite of ownership: "I own this box" implies "This box belongs to me."

Whether you use "belong" or "belongs" simply depends on whether the subject is singular or plural. "It belongs" but "They belong".

You can use it in the simple present or in other tenses. "This belongs to Bob." "This will belong to Bob" (perhaps after he has proven himself worthy). "This belonged to Bob" (but it doesn't belong to him any more). Etc.

You don't say "is belongs", just "belongs".

I am a native AmE speaker.

I am at the Hearing Officer's convenience.

This is close to the idiom "at your convenience" and it sounds like your timing is convenient in reference to the Hearing Officer's schedule. You may need to speak to them, but you can wait for them to have a free moment instead of needing to schedule an appointment.

I am at the convenience of the Hearing Officer.

This one suggests that you can be of assistance to the Hearing Officer. You are available to run errands or do trivial tasks to make their life easier, much as a personal assistant would.