(1) I think if you get too far into that kind of thinking, you'd never do anything in case it cancelled out something else you might want to do in the future!
First off, a significant chunk of jobs don't ask for you to have a degree in a specific subject; they just ask for you to have a degree. Any subject at all will open up a lot of doors that would otherwise be closed, on top of the engineering jobs it could specifically lead to.
The way funding works, you get a loan for one degree (plus the 'false start' I said about before). Beyond that, you won't get funding for another one, unless you do one of the very few subjects (e.g. nursing) that are paid for by the government. But, don't worry about it! You're thinking about what will happen if, in the future, you get a job that's so great and absorbing that you want to get a degree in
that area instead of engineering... well, even if that happens, maybe your employer will be prepared to pay for you to get one, if it's that relevant; or maybe you'll be able to go straight on and do a Masters in whatever it is. Personally, I'm in the process of doing a second undergraduate degree part-time, just because I thought it was interesting. That doesn't mean I regret my first one, or feel I did the 'wrong' subject; I just decided I'd like to do another one, so I am, and I'm paying for it as I go. Nothing's set in stone.
(2) My advice, as far as those kinds of things go, is just to go for it. I know Engineering can have a fairly full (and quite variable) workload, but even still, I think it's likely you have at least as much (if not more) time available to you now than you will when you start full-time work, whenever that day comes. (My first degree was one where I spent eight hours a day in uni most days, and I still had
way more free time then than I do now).
(3) You might want to read
this book if you're thinking of writing a novel - it's the book that goes with
Nanowrimo (but you obviously don't have to wait 'til November to do it). There are pros and cons to approaching things in that way, obviously, and I'm not necessarily advocating you attempt the 30 day thing (although it can be fun, and generates things you would never have written otherwise) but a big (and useful) part of the philosophy behind it is that the whole 'one day, I will write a novel' thing can act against you if you worry too much about how, and when, and how to make it perfect, and so on. (There's a bit in the book where the guy explains how he had a whole plan to take three months off work in which to concentrate solely on writing, going to considerable effort to rearrange his commitments and free up that time - and then he didn't get any writing done at all!) Just starting, and giving yourself permission to write a dreadful first draft, is actually very freeing, and you start achieving things and making progress you never would have otherwise. Ditto with everything else, really - learning a language, or to play an instrument, is a series of tiny steps.
(4) My advice is, don't think in terms of 'when will I definitely have time to devote to this on a consistent basis?', just think 'I've got half an hour, I can practise that chord/learn five new words/write a couple of paragraphs.'