A to the Q:
Ekaggatā refers to being concentrated on one object, or on one point, especially in samādhi. So in that sense, the terms are definitely synonymous.
That said, samādhi is a broader term: it refers to overall meditative stability and concentration, stability or concentration that is not necessarily pinpointed on one meditative object. There are powerful and beautiful forms of samādhi that take in the overall experience of the moment, or move between the many different objects of attention coming and going. There are technical terms for some of the different kinds of samādhi, which I won’t go into here. The territory at of samādhi is varied & vast. Ekaggatā is one part of a much larger landscape.