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In-between. A related double major can improve your understanding of a specific speciality, but no more than simply taking a few extra courses would. A major is meant to be a reasonably complete set of courses to pursue a particular career - a second major prepares you for two careers rather than extra-preparing you for the first one. Remember that engineering as a profession is pretty specialized, so it is highly unlikely that you will find yourself in a position where the breadth of two majors is really necessary or even helpful.</p>
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No, they don’t, just like in other fields. There are several reasons for this - most people don’t want to be in school that long, most people don’t want to do research, the career incentives are not significantly better, the long-term pay impact is about zero…</p>
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No, many engineering PhD’s go into research labs, government work, etc. In some schools, this is actually a much more typical path than going for the professorship. I myself do not have any intention of going to a university - I will be working for my company in a research role.</p>