A subjunctive as the main verb, often in a question, can mean "would, could, might" (potential) or "should, am I to" (deliberative).
Potential: the present or perfect subjunctive means "would, could, might", with no difference in time; the perfect here is not past: quis … abstulerit? "who could take away?", qui possit? "how could it?" (qui here is an old word for "how").
Deliberative: a question about what should be done: cur … metuat? "why should he fear?", quid faciam? "what am I to do?". The negative of both is non.
Most such questions in Marcus are rhetorical: "who could …?" means "no one could", and "why should …?" means "there is no reason to".
The perfect subjunctive looks like the future perfect except in the 1st singular (fecerim, fecero); as a main verb with no future in view, take it as potential. The imperfect subjunctive gives a past potential: crederes "you would have thought".