Nick,
Can't comment on how long short-haul pilots are away from home for, although there are plenty of people around here who will know.
As for instructing, there would be no problem with becoming a "career instructor" - many people have done this. Most instructors do not have any airline experience. In fact, many instructors are building hours so that they can move on to the airlines once they've got the necessary number of hours.
At the moment, getting any flying job is difficult, even instructing - many pilots who lost their jobs over the last year or so are turning to instructing to make a bit of money. But things are improving - see the many other threads on this forum to catch up on the ongoing debate.
Apart from that, the main issue with instructing, especially career-instructing, is that there's not much money in it. You won't be making much more than minimum wage, and you'll be working long hours, especially in the summer. Personally, though, I think it would be a great way of making a living, as long as you didn't mind the low wages!
The first thing you should aim to do is get a PPL. You can do this in your spare time, without quitting your current job. By the time you've got a private license, you'll know whether flying is something you really want to pursue.
You'll also have enough knowledge by the time you complete your PPL to decide whether to get an ATPL or not. Most people seem to think it's best to do the ATPL exams even if you only want to get into instructing, the reason being that if you change your mind later on, you can complete the ATPL without too much hassle, whereas if you'd started with just the CPL exams you'd have to do a whole load more exams to get an IR and continue with your ATPL.
Good luck!
FFF
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