I don’t actually want to do anything that defends Josh Duggar. However, I can not condone in general solitary confinement of the nature you described as being part of our system for anyone who does not present a very large danger to other inmates and prison staff leaving them with absolutely no other choice for the safety of others. I encourage you to read up on its effects and just how terrible it is it really is akin to torture. I don’t want to live in a society that condones regular torture of its prison population.
As to him being shanked I doubt it, unless something new comes go light he will almost certainly end up housed in a medium security prison with a large percentage of other sex offenders. Getting shanked is more of a maximum security prison issue. In medium security people generally don’t have a history of violence and are not going to shank someone and risk their own release.