Is Mrs. Robinson's seduction of Benjamin actually a plan by Mr. & Mrs. Robinson to prevent Benjamin from marrying their daughter?

Mrs. Robinson is the original cougar, so to speak.

She seduces Benjamin simply because she is tired of her marriage and wants a fun sexual encounter. Mr. Robinson was most certainly not aware of this.

From the Wiki:

Mr. Robinson, [once he learns about] his wife's affair with Benjamin, goes to his apartment in Berkeley and berates him, threatening to have him prosecuted to the fullest extent of the law, should Benjamin ever come near Elaine again. In the meantime, Benjamin has tried to tell Mr. Robinson that his wife has manipulated him into having an affair with her. But Mr. Robinson is also skeptic and refuses to believe him, and then he forces Elaine to drop out of school and takes her away to marry Carl. She leaves Benjamin a note, saying that although she loves him, her father's anger would prevent the family from ever accepting him as Elaine's husband.

Mr. Robinson was outraged once he'd learnt of Benjamin's actions, blaming him entirely for it.

Rebecca Neumann in an article on the film gave some explanation as to Mrs. Robinson's actions:

The film places significant emphasis on Mrs. Robinson's past as the motivation for her actions. As a young woman, an unplanned pregnancy forced her to give up her dreams, drop-out of college, and marry Mr. Robinson. Years later, her loveless marriage and empty nest bore and embitter Mrs. Robinson despite her luxurious life-style.

So no, Mrs. Robinson's seduction of Benjamin was most definitely not a plan to keep him and Elaine apart. It just ended up looking like that due to Mr. Robinson's reluctance to ever let Benjamin near his family again.

It may have been a plan by Mrs. Robinson, only.

Mr. Robinson was the business partner of Ben's father, and initially welcomed Ben's dating Elaine. When he told Ben to "sow some wild oats," he may have envisioned Ben doing that with Elaine, but certainly not his wife. Later in the movie, he calls Ben "degenerate."