4. What are some related occupations if I lack some of the necessary skills for this job?
If you can't write but have the technical skills, look into software or Web development, or other fields of engineering or technology. If you're good with people, look into schools where you can train others.
If you have language skills but don't like the actual writing part, look into editing and proofreading, whether technical or not. I actually enjoy editing and proofreading more than writing in many instances, and there is work out there in both technical and non-technical fields.
If you can write but lack the technical acumen (and don't wish to acquire it), consider editorial positions for newspapers, trade magazines, general circulation magazines, or book publishers. Also look into marketing, publicity, public relations, and advertising -- not just in firms that do these things as their main line of business, but at companies that have divisions in-house, even though they do something else primarily. Almost any organization needs people good with words.
5. What is the salary range?
Middle range. You won't get paid as much as an engineer or high-level manager, but I seem to be able to charge $50 an hour without any complaints. In theory, that would gross me $100,000 per year or so if I worked full time, but it's rare to get consistent billable 8-hour days, every week year in year out. I only work part time, as I mentioned, so I'll probably bring in $45,000-50,000 or so in a year, but that's before taxes and considerable expenses. Working full time, you should be able to NET at least that much if you're good. But that's a guess.
6. How long does advancement usually take, and where does it usually lead?
I have no idea, really. Within companies, tech writing departments are small and (as with all tech fields) turnover is reasonably rapid. If you stay around long enough, you're likely to end up in charge, but not of that many people. Unless you want to go into management instead of writing, that's about as far as it goes, to my knowledge.
Working freelance, the benefits of experience are that you get to pick better work -- stuff you like more -- and get to reject more stuff you don't like. Plus you don't have to work as hard hustling up work, since you have repeat clients and word of mouth working for you. And, over time, you can charge more, to a point.
If you're looking at a skyrocketing career into the upper reaches of fortune, fame, and wealth, tech writing is probably not the way to go. If, however, it is what you like to do, then you can get to do more and more of what you like over time, and live comfortably doing it.
7. Does a typical worker have a set schedule or are the hours flexible?
That depends on whom you work for. As a freelancer, my schedule is as flexible as I want it to be, or as my young family allows (which is why I'm writing to you at 11:00 p.m.!). Companies may have strict working hours, or very lax ones. Most are on the flexible side, since tech writing is the kind of job where you just need to get stuff done on time, regardless of how you get there.
That said, it's good to have core available hours for meetings and such. Being available only after 9:00 p.m. or on weekends isn't very helpful if you want to do work for more normal people.
8. Do you work a lot of overtime?
Again, it depends, this time on what you mean by overtime. Usually, I work a lot of nights and weekends, but that's because I DON'T work a lot of weekdays, since that's when I take care of my kids. I don't work 40-hour-plus weeks by choice. But when deadlines loom, I sometimes work a lot more than that, and at very strange hours.
Tech writers are very rarely unionized, and are considered technology professionals, so it's rare to get any overtime pay, even if you are working what are theoretically overtime hours.
In my current contract, if I'm behind where I need to be when the deadlines get close, I may put in late and extra hours. But those may very well be because I didn't work that hard early on -- so it will be my own doing.