Inter Milan will not let that happen, it will definitely maintain at least a draw for Inter Milan to qualify.
Who would have thought that Cadiz had a 1-0 lead in the first half even though in the second half it would reverse its situation, it would not be surprising anymore.
Back to the Champion League match I don't feel Atletico Madrid will qualify their chances at home are so great but I judge how from Inter Milan's current performance which is so consistent in the Champion League and Serie A, their minds will be distracted to stay focused on the champion league match.
Atletico Madrid's performance this season was very poor and even when they played at home they were not able to maximize every opportunity available, in the first leg, I think Inter Milan played better by controlling every line but indeed Atletico defense at that time was fairly solid so as to reduce the aggressiveness of the attack that InterMilan did throughout the game, In fact, I think Atletico Madrid playing defense at intermilan home is something that might destroy them in the second leg.
The second leg will indeed be played at home to Atletico, but i think there is no change for Intermilan to not be able to win the game, at the moment the advantage is on Intermilan side and as you said they only need a draw to get them through to the next round, atletico may hope to turn things around when they play at home, but with a performance like now it might be difficult for them to make it happen, as with any other team that plays a relatively attacking scheme, of course Intermilan will do the same at Atletico Madrid home later, because it seems that attacking is a good way to break the concentration of Atletico Madrid players as they did in the first leg.