The concern for me is if he is in the general prison population whether he will continue to influence people. I suspect in the enclosed limited environment of a prison "radicalisation" would be much more effective and easier to achieve.
Indeed. He belongs in a supermax or equivalent facility.
Barring that, deportation to international waters is a good idea. At least 100 miles from the nearest land and shark-infested, preferably - I don't like to take unnecessary risks.
McHrozni