IQ Sampling and Oversampling Differences? *Confused*
I think you're confusing IQ sampling with oversampling.  Your first scenario is oversampling, or taking twice the number of samples on a real signal.  Other than stating the sampling takes place 90 degrees apart (it would actually be 180 degrees), your analysis is sound.

In the second section, you describe IQ sampling.  This is usually done by mixing a signal with a nonzero center frequency to baseband using two mixers, which multiply the same signal by a sine and a cosine, respectively.  The two mixing signals, separated by 90 degrees, are generally output from the same oscillator.  The outputs of the two mixers are separately sampled to get the I and Q components of the sampled stream.  This is typically represented as a stream at the original sample frequency but with two values (I and Q) for each sample.

IQ sampling essentially allows one to differentiate between positive and negative frequencies in the baseband signal, so it does in fact double the useful bandwidth of the signal just as oversampling does, but in a different way.  Instead of doubling the Nyquist rate, it revitalizes the band from -Fn to 0.