Can multiple sclerosis be screened for prior to any signs or symptoms?

Not easily. There are small studies looking at the risk of developing early stage MS (clinically isolated syndrome) after getting an MRI (for non-ms reasons) which has changes very suspicious for MS lesions. The risk is low but not zero, with a minority of cases going on to have a neurological event which would put them into a clinically isolated symdrome category. Screening MRI are not currently advised.
Sure. We call this radiologically isolated syndrome, when an MRI shows characteristic lesions, but the patient has yet to become symptomatic. But this is a fortuitous finding, not from screening, and currently, seems not an cost effective way to predict ms.
Yes. But it is not recommended since you are exposed to unnecessary radiation with an MRI of the brain and spine as well as a spinal tap. Nothing in medicine is without risk.