If you don't have an acceptance by the end of December,some schools that haven't made up their minds ask you to send in your grades for fall.
I have this problem too and my professors have been very accommodating. They have all gone out of their way to accommodate my interviews.
You have to go talk to them and just be apologetic about missing their class. Most are really interested in knowing where you are interviewing and if they know people at the school give you an idea about the research environments in those schools.
I go to school where if you put in the work, it isn't difficult to get an A so I am still doing well on tests and that makes the professors more sympathetic to me never being in class or lab. They pretty much are waving parts of the class for me. Go talk to your professors, I'm sure they've dealt with this before.
There are lots of risks to getting poor grades. If you don't have an acceptance by december, you may need to send your grades in if the school asks. If you have a 3.6 now, you could fall below a 3.5 and depending on your school, risk not graduating with honors. There are people who have as many interviews as you do and still don't get in and so on....
Until you have that acceptance in your hand, don't shoot yourself in the foot by getting poor grades. Play it safe, do what you can to get good grades.