Leverkusen is quite comfortable at the top of the Bundesliga standings with a 10 point lead so I think that way they don't have any pressure in the Bundesliga so they can focus on their Europa League matches.
I don't think what you say has a strong reason, it's impossible for Leverkusen to put aside the Europa League for the sake of the Bundesliga which they are almost certain to get.
Indeed. Bayer Leverkusen is leading with 10 points gap. The must be confident with the chance to win the title of Bundesliga this season. Sure, they won't feel relaxed, they will always focus on winning every match in Bundesliga. But it won't be a very big burden anymore because they are getting closer with the title. There are only 8 remaining matches, I am sure it won't be so difficult to get points in each the match. I see the substituted players of Leverkusen performed well, too. So, Xabi Alonso won't be so difficult to rotate the players both in Bundesliga and Europa league.
Roma does have quite good strength and has experienced very significant improvements since De Rossi's arrival, but I don't think it's easy, even in the quarter-finals against AC Milan, I don't think Roma's chances of qualifying are much greater.
It is not sure that AS Roma can defeat AC Milan. Many people assume AC Milan is still better than AS Roma. I can understand that people favor AC Milan because AC Milan has a better experience and history in international matches. If we learn the current performance of these teams, I think both AC Milan and AS Roma has almost the same chance.