Mulkari

In Latvia limit is 125 g of fuel and 500 g of total mass, above that some sort of certification and launch permits is probably required. However as long as you launch in uninhabited areas far away from airports I bet you could get away with fairly powerful rockets. I know there is a guy in Latvia who built and launched a rocket that easily goes 3 km high approaching the speed of sound and it never got him into any trouble with law. A model rocket is fairly useless weapon, it is unguided and have no useful payload capability and range. I bet someone skilled in model airplane building and programming could build a crude cruise missile with few kg bomb as payload from model airplane parts guided by cheap gps receiver and smartphone. But why bother with it if for the same effort you could build a half ton car bomb.

Interesting that Soviets managed to develop relatively cheap staged combustion engines. US made SSME which also uses staged combustion is so expensive it would never be profitable to use in expandable rockets.

Well, it could work that way if it is possible to engineer the shield that way without compromising durability. Although you would still need chemical engines for roll control. However since the ship would need RCS thrusters anyway it is not a big deal to add some a bit more powerful RCS for roll control. Since full scale testing of propulsion system on the ground would not be possible assembly and testing of a prototype in space would be a must if a ground launch is seriously proposed. What happens if there is a dud nuke, what if for some reason blast vector is not properly in line with center of mass, how quickly it is possible to recover from spin and continue on course. How well the shock absorbers and blast shield stand up to hundreds of explosions. All of that and a lot more would have to be known before a successful ground launch could be attempted. Launching without full knowledge of how the whole system behaves and possible failure modes almost certainly would mean N1 on steroids.

I'm not sure if it was mentioned somewhere, but how the Orion rocket would be steered during powered flight especially in case of ground launch where atmospheric turbulence would try to push it off course? Chemical rockets are steered by moving an engine nozzle. I doubt it would be possible to do the same to a Orion blast shield. Maybe some small chemical rocket engines also would be included to provide attitude and roll control.

Fallout is manageable, there were hundreds of nuclear tests done without any regard to fallout. Real security issue would be a failure while still suborbital and ship crashing for example in some lawless country in Africa, good luck in retrieving the bombs remaining on board. At first a scaled down test model would have to be assembled in space so the propulsion system could be fully tested without a possibility of crash in case of failure. Likely multiple in space propulsion tests would be required to understand possible failure modes and to refine the design for full scale ground launched version.