Passband vs Baseband Bandwidth
You can unify the definitions for baseband and passband bandwidths by saying that bandwidth is only measured at positive frequencies. So for low pass signals, the lower frequency is zero and the upper frequency is the cut-off frequency. For bandpass signals, you have the difference between upper and lower cut-off frequencies (at positive frequencies).

For complex signals, a reasonable definition of bandwidth should take into account that the negative frequencies are not redundant anymore. So a complex low pass signal extending from $f=-B$ to $f=B$ should have bandwidth $2B$.

Concerning the choice of sampling frequency for bandpass signals, take a look at this answer. It is not true that a sampling frequency equal to twice the bandwidth is always sufficient. Yet you can usually do better than just sampling with twice the upper cut-off frequency.