Henrietta
The agents are taking advantage of you, they obviously care more about the convenience of their prospective buyers than your rights as a tenant to enjoy peaceful occupation of the property. Under Victorian law they must give 24 hours notice at least, and only visit between certain times. The Tenants Union has a series of fact sheets, one here is particularly relevant:
https://www.tuv.org.au/wp-content/uploads/2014/03/landlord-is-selling-FS-2013.pdf
In regard to the rent reduction offered, I guess that would have been verbal only. But it would leave a bad taste in the mouth if not honoured.
In regard to the photos you would possibly have a case to complain the the Privacy Commissioner. If there any photos being used in advertising which you object to, I’d be writing formally to the agent to withdraw those ones from their campaign. After all publishing such details could leave you open to security concerns when placed in the public domain.
You need to put your foot down and write a strong letter to the agent objecting to your treatment, keep a copy.