Yes but if you only simulate a consciousness there is no nesting, otherwise on what does the consciousness run on? That’s the whole idea behind as you lose fidelity with reality the simulations becomes less capable until it’s unable to simulate anything.

Note nesting isn’t necessary for the probability argument to be true just many simulations. But the vanishingly small likelihood of not being a simulation depends on the nesting idea.

Again, yes, it’s possible what we observe is a simple simulation of experience but I don’t think this is the “almost certainly a simulation” argument.