In the salt belt aluminum parts under the car can sometimes be trouble when it comes to corroded fasteners. When I do a job like this I always figure in my head what will I have to do if this or that happens, I always have a plan for the worst case scenario.I think what Trav and others are saying is that if the diff cover bolts break off then you have to get those out and it's a bigger mess. You are already having issues accessing the drain plug and without welder etc, you may get into larger problems.
What are you comfortable with? Maybe it's best to get the new cover and drain bolts and get a shop to remove the old stuff and install the new and make sure they put antisieze etc on the new ones and don't over torque anything.
In this case it would be a bolt broken I cant get a drill into because it is obstructed. These would probably be the upper cover bolts and those are the ones I break loose first, If one snaps it can still be driven to a place with a lift.