if you have that level of engineering expertise, just skip the whole "steal nuke" part and build your own with stolen enriched Plutonium.
I assume defeating the security component would be easier and less costly for a bad actor to accomplish than to design and construct the whole thing from raw materials.
Yes that was considered all the way back in 1945, which is why nukes back then had multiple interlocks to prevent such a thing.
You mean before the mass production of many nukes became a priority, and some brilliant manager probably decided that eliminating redundancy would let them produce more nukes on their budget, and software automation of ICBM launch would reduce the amount of manpower required to maintain readiness?
Now, point out to me where the possibility exists that a faulty electrical component in the unit's electronics could detonate the bomb?
Undetected non-simultaneous failures of multiple different electronic components.
Not really. The starter switch in the ignition key switch, on my car at least, has separate wires going to power the ignition, fuel pump, and the starter solenoid.
There are a lot of cars nowadays, you can start with a push of a button in the car, or on a keyfob remote.
"Starting when in gear" is not mechanically prevented, it is electrically prevented.
There are Electric-Unleaded Hybrids now that have a continuously varying transmission, so there's no such thing as "In gear"; Drive-by-wire systems, where Brakes, Throttle, and all the settings are controlled by computer.
Just like not all cars have the same design, not all nukes have the same design.