Assuming the engines to be perfectly reliable (100% reliability no matter the conditions),
With such an assumption then we can presume that folks will fly in single engined airliners - why waste all that money on more than one really needs ?
It was hypothetical.
In other words, if the engines are removed as a concern, then what is next? You addressed this though in your next part of your reply.
Put another way, of ALL diverts of aircraft flying hours away from the nearest suitable, how many were due to something NOT engine related?
Don't have any figures to hand. However, systems other than engines are the driving concern in recent years - multiple electrical failures, on board fire are but two which spring to mind.
Fire is a strange one; it is thought in a lot of cases they couldn't reach the surface in time even if the airport was ideally located (e.g. Swissair Flight 111).
what is the chance of a 4 engine suffering dual-engine failure compared with a twin suffering dual-engine failure?
Somewhat higher, I suggest - two engines are on the same side. If one lets go in a big way, there is a rather higher probability that debris will take out the adjacent engine rather than (the) one on the other side. Plenty of examples around to back up that thought.
That is a good point. Even the Qantas A380 ended up being unable to control #1 after the #2 let go, despite #1 itself being undamaged.