I would NOT skip a Magic Morning/
Disneyland Early Entry to get into Cars Land. Recently I've seen several days (in person and per DCA Today's Facebook) indicating they are doing planned maintenance on Radiator Springs Racers...the maintenance seems like it may mostly fall on days when DCA isn't offering early entry. If that happens, obviously you couldn't ride RSR when you got in, and the other attractions of Cars Land may be quite busy as well (since people would rush to Cars Land, find RSR is closed, and then do the other attractions).
You can still get Fastpasses if RSR isn't open yet, but the return times would have to be no earlier than the time the ride is scheduled to reopen. So, your the return time would probably be just as late or even later than if you'd come in as an off-property guest on an Early Entry day.
I wouldn't worry so much about the rest of Cars Land. Reports are that Luigi's wait isn't too terrible anymore; you should be able to ride it in 30 minutes with any luck. Mater's seems to be shorter, maybe 15-20 minutes, but if you like Mater and/or hillbilly-style country music at all, the wait for that ride is actually enjoyable. Or at least, I enjoyed it and now regret only riding it twice because the line was "too long" at 20 minutes.
That said, since that day is at the END of your trip, you can make the decision on Tuesday.

Have a great trip!