In your sentence, “return to” or “participate in” would create meanings that are basically opposite to come away from.
“Return to” is basically the opposite of “leave”.
“Participate in” would refer to the time during the seminar. It does not refer to the completed seminar-- after which you would be able to leave with the result of the full seminar (a “better understanding”).
Replacing “come away from” with either “return to” or “participate in” does not create a sentence with the same meaning.
A second point is:
The words returning and participate also wouldn’t be grammatically correct in the sentence.