How can a anybody other than those permitted by you access your FB material, if you chose your FB privacy options correctly.
In any event, if the pictures are such as cannot lead to a reasonable possibility of extra-marital affair there is nothing anybody can gain from them [I was dying to use a more desi phrase for it, but let it be] (and extra-marital affair ain't a joke of holding hands with opposite se.x buddy); if they are such as to lead to such assumption or conclusion, you had no business to publicly share them in the first place and you deserve it.
And so far as activity log is concerned, I don't think ANYBODY except the user himself can view it (somebody correct me if I'm mistaken).
There is no breach of your security or stalking to form any offence under the IT Act. Yes your privacy was breached...sorry you don't live in Europe and there is no law to give you an actionable right for breach of privacy.
Nobody can authenticate WhatsApp messages as primary evidence at the moment in India for the reason that they are encrypted end-to-end and nobody except the sender and the recipient, not even WhatsApp Inc., can intercept or decrypt them, A screenshot may be proved as secondary evidence.