For those who are searching for a car to rent, I can also recommend VitaCar from my experience. We had a problem with our first car (engine stopped working sometimes), but they quickly managed to change the car to a better one. Excellent price and attention to customers.
You can find bus times here www.ktel-lefkadas.gr/…
This timetable goes to the end of May, in summer the service is more frequent.
Just a tip about using the forum,as your question is about buses from Nikiana to Lefkada, you would probably get more replies if you started a new question rather than adding your question to an old thread about car hire from Nidri which people might not bother to read as it was answered last year!
By the way, the buses are quite reliable. The summer timetable probably won't be published until May.