Wordpress website | Constant Contact API

We just implemented Constant Contact API  within our wordpress website. A few questions if anyone is kind enough to respond to:

1: Does data (our contact list) synchronization works both ways now? Does adding or deleting a contact on 1 side affects the other side and vice versa? 

2: How long will it take for the data synchronization to happen? Is it instant? 

3: One of our new staff members installed "constant contact" wordpress plugin by mistake without letting me know. It affected the API (which makes sense) so I deleted it and now API works again. I just want to be sure that since we have implemented API, we should never use the plugin (no need for it regardless after API)? Correct?

Thanks for any response. KTG