I think Felicity died and is in the grave.
Oliver stayed behind to grieve by himself. Later he is shown getting into a limo and Felicity is inside and crying and upset.
I think that Damien Dark found out about the Green Arrow - Felicity connection and to keep Felicity safe, they had to fake her death.
That is why Barry Allen shows up to the funeral. In one of the shows, it was said that Star City is 600 miles away from Central City. Barry Allen is only close to Felicity. He would not make the journey for Diggle, Thea, Oliver's baby mother or Oliver's son.
In past episodes, Felicity gets upset when she learns about Oliver's son and they break up. She would not be that upset if it were either the baby mama or the son.
If it were Diggle, then his wife and child would be the last ones to leave the grave. If it was Diggle's wife, then it would be Diggle and his child the last ones at the grave.
If it was Thea, then Malcomn Merlin would be the last person at the gravesite.
If it was Laurel, then her father would be the last person at the gravesite... if it was her father, then she would be the last person at the gravesite.
Barry Allen would only come if it were Felicity. He would only come to make it look real. He missed the funeral fighting Zoom and Oliver was not upset. Barry couldn't go to the limo and say hi to Felicity because somebody might see her.
Who is driving the limo? Diggle.
Who is in the grave has been very interesting,... but I am just waiting for that other suprise to happen that they have been teasing all season. That one is very more exciting.