I don't think Roma would be able to qualify to the semifinals, because they are playing with a team stronger than them. Milan have been quite consistent recently and I believe that they also have the Europa cup finals in heart. It will be a shock to me, if Milan is kicked out by Roma in the quarter finals.
Roma is doing pretty good, ever since the arrival of De Rossi, but that does not mean that when they face a strong club like Milan, they would be able to be victorious over them. Roman's strength is limited. Since Di Rossi took over from Mourinho, Roma have not won a big club if I am not mistaken. Let's not underestimate Milan who is sitting on the second position, while Roma is 5th on the table in their domestic league. Assuming, it was a different league, I would not doubt Roma.
Roma are playing well recent matches with De Rossi but AC Milan are not bad and about experience in Europe, AC Milan are more experienced than Roma and also are more successful than Roma.
Results from their past matches in Serie A show that AC Milan are stronger than Roma in long history and it is another advantage of AC Milan before two quarter final matches with Roma in Europa League.
In a latest match with Lecce in Serie A, Roma shown problems in attacking when they did not have Dybala and Lukaku could not find opportunities by himself. To play well, Lukaku needs support from a creative and skillful forward like Dybala. AC Milan can set up his tactics to control Dybala and minimize danger from the forward, and if they succeeds, Roma will lose like half of their attacking power.
Liverpool chances of making it to the finals of the Europa League is more higher than that of Bayern Leverkusen making it to the finals. Checking from the match fixtures, the matches and teams that Bayern Leverkusen are going to meet are more tougher than the ones that Liverpool are going to be playing against. The AS Roma vs AC Milan match will be the toughest in this quarterfinals and any team that qualifies amongst them will be facing Bayern Leverkusen which makes their chances of making it to the finals under big probability.
Liverpool have easier branch than Leverkusen because if Leverkusen win in quarter finals, they will meet either AC Milan or AS Roma in semi finals. Roma and Milan are not easy opponents for Leverkusen in semi finals. In Liverpool branch, they have easier opponent in semi finals so chance for Liverpool to get through the final match is higher than Leverkusen.