Hi Priyanka,
Answering to your question (6), you don't really need to worry about the sampling frequency of ADC as those are internal things and taken care of according to the bandwidth specified by the user (host PC). The sample rate (bandwidth) that you mention in GRC is the rate at which the 'USB Control' transfers samples to the host PC (refer attached image). Coming to your question about Nyquist sampling rate - The input of ADC is at IF range and not at baseband. So a sampling rate of twice the bandwidth of IF signal is enough (instead of twice the maximum frequency). You can find more details via internet search with terms like 'bandpass sampling' or 'undersampling'. The theories in courses don't talk about this as they assume that input to ADC is always baseband but in practice, for high carrier frequencies, undersampling make things easier.
For the question (7), I don't know much about GPS signals but I can say that you need to consider both I and Q samples as the samples from RTL-SDR are raw samples and hence you need the whole complex samples to demodulate (if it is not BPSK modulated) and to perform other signal processing like synchronization.
Hope this helps.
Regards,
Sakthivel