Are ferries still going to the Botanical Gardens ?
Nazirgunj Ferry Ghat is the nearest ferry service near Botanical Garden. Botanical Garden also has a ferry ghat, but I am not sure if it is operational for commuters.
Opposite of Nazirgunj Ferry Ghat is Metiabruz Ferry ghat on Kolkata side.
Metiabruz is better option if you are coming from South Kolkata only.
It would appear to be possible to reach Botanical Gardens (Nazirgunj) from Fairlie Place by changing ferries at Metiabruz ??
Absolutely. There is a service from Fairlie Place to Metiabruz.
Ask at the counter if there is any direct ferry to B. Gardens.
Even if there is not, your plan seems perfect.