Yes. I suppose that probability to get such bad random distribution which makes SL search to be linear is similar to probability of moving all oxygen atoms to only one side of your room.
If the adversary find a bug in your system, such as not so random "random generator", then it can bring it down. But if the random generator is reasonable, then you can expect it to behave properly on all inputs.
Yes, b+trees are more performant for reading compared to skip-list because of better data locality on memory/disk. But SL better at insertion and sequential reads. 255 GB is a simple trade-off between disk space required per record and the ability to manage large data-sets since SL is a not very space friendly data structure) Although I think 255 GB is good enough in many use cases.
