Without more context, here are my best suggestions: All in all, from what has been discussed above... or All in all, from what I have discussed above... To sum up, from what has been discussed above...or To sum up, from what I have discussed above...

to answer specifically your third question, the usage of 'have + past-tense_verb' connotes the indefinite recent past(while 'had +past-tense_verb' connotes the definite past) (roughly). Using the verb 'to be', in this case the particular word "been", denotes being in a certain state. So, you could not say 'been discussed' because you are/were not in a state of being discussed, but rather a state of discussing. Because the act of you discussing would be fresh in the minds of the listeners at the point of saying this sentence and could easily be placed by them in the recent past, you would most commonly hear 'I have discussed' (emphasizing recent past) or 'I have been discussing' (emphasizing your state of discussing in the recent past and the present. ie: you are essentially continuing to discuss what you were discussing ie: remaining in a state of discussing)