Option 5:

Choose flights on CRJ-700 planes with explus seating (28 E+, 32 E-), booking after E- seats are all taken (so that you won't get a seat assignment). Upon checkin, you are likely to get an randomly assigned E+ seat, even if you decline the buyup.

Of course, if the flight really is completely full and then some, not having a seat assignment increases the risk of being IDBed.