Excellent questions!  Let's go with the first one first.  As far as I can determine, there is no evidence that JFK knew about and approved of the Diem assassination, but that really doesn't let him off the hook in my opinion for the assassination.  It merely reduces the degree of presumed culpability.  I say presumed culpability because short of a divine insight we would never know enough on the matter to know what actual degree of culpability, if any, JFK may have had before Almighty God in the matter of assassination.

As I said, I think it merely reduces the presumed culpability of JFK for the assassination of Diem.  JFK definitely knew about the coup desired by certain Vietnamese Generals and knowing that he was in a position to halt the coup or -- at the very least --  attempt to halt the coup, he did neither.  (See the below audio/transcript link for some direct testimony of the president himself concerning the coup.)  It would not have taken a rocket scientist or a POTUS for that matter to know that a coup by the Vietnamese Generals against the Catholic President they did not want to rule them might very well result in a dead president.  Assassination of the overthrown leader, if not always inevitable in a coup, is certainly quite common -- especially in a coup carried out during time of war.

Some of what JFK recorded (http://www.wbur.org/news/2013/11/21/kennedy-records-private-vietnam ) on November 4, 1963 concerning the military coup which violently overthrew a fellow Catholic President: "Monday, Nov. 4, 1963. The — over the weekend, the coup in Saigon took place, culminated three months of conversation about a coup, conversation which divided the government here and in Saigon.  I feel that we must bear a good deal of responsibility for it, beginning with our cable of early August, in which we suggested the coup."

A good source pointing to incrimination or lack thereof for JFK's alleged involvement in the coup resulting in the death of President Diem depending on your interpretation of same is seen here: https://nsarchive2.gwu.edu/NSAEBB/NSAEBB101/  Of particular note is the AUDIO CLIP found between DOcuмENT 18 and DOcuмENT 19.

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As for your second question, the actual physical assassination of Diem was by all accounts that I've ever run across carried out solely by Vietnamese who were in no way involved with the assassination of JFK.  I strongly suspect, however, that there were at least some people who had varying degrees of knowledge of the planned coup of Diem and who also exerted some varying degree of influence in pushing for the coup to go forward with the very likely assassination of Diem in the process who also had varying degrees of knowledge and culpable influence in the moving forward of the plan to αssαssιnαtҽ JFK.  I think some of the names of these people are included in the names of the people found here: https://nsarchive2.gwu.edu/NSAEBB/NSAEBB101/

On the issue of RFK's Catholicism, here's a telling quote on his and JFK's visit to the St. Benedict's Center in the 1940s:

"Members of the famous Kennedy family were known to visit. Jack was running for Congress when he first came to the Center, and surprised Father by being able to recite from memory passages of one of his essays. Jack Kennedy came to several lectures and was always gracious and respectful towards Father, who once told him, “Maybe someday you will be the first Catholic president!”

Bobby Kennedy came once as well. However, he was arrogant and confrontational. Father suffered him patiently, until he flippantly sniped, “I know more Protestants who are going to heaven than Catholics!” “That’s not the way to talk to a priest,” Father retorted, and directed the young Kennedy to the door."

St. Benedict Center source