●1) Looking back, it seems that most of my childhood consisted of chilling on a sofa, while my parents were working / worked / used to work away writing.

I think both "were working" and "worked" would be correct, but "were working" flows better since the word "chilling" was in a past participle.

2) From a six-year-old's viewpoint, their work seemed / used to seem awesome, and my parents were / used to be very laid back about stuff.

I think in general using "used to be" implies that they stopped at some point within your childhood and somehow had a personality change, versus the fact that you grew up and they no longer needed to be.

3) I used to share / had been sharing a bedroom with my sister Charlotte for years and then next thing I knew it was just me.

I think "used to" would work better. This is really down to my own personal preference and honestly both work perfectly!

4) Charlotte and I would talk / used to talk for hours about our plans for the future.

Again, both are correct and it's down to how you want to express yourself.

They took the time to talk to us and see how we were getting on and if we 5) had / were having a tough time at school they would give us a special treat.

Although "were having" and "had" are both correct, they still technically mean two different things. If you said "We had" then that would imply that maybe something happened bad once, like getting a bad grade on a test. Whereas "were having" would be if an ongoing situation was bad, like being bullied. Bullying doesn't stop after a day and it certainly doesn't stop once your parents know about it, so "were having" would be proper in a situation like that.

They 6) didn't spoil / were not spoiling / wouldn't spoil / didn't use to spoil / hadn't spoiled us though!

The two remaining forms are the only ones that are correct. "Didn't use to spoil" has a bad flow to it, and the "use to" is redundant.

●We finished our sixth form studies a couple of months ago and soon after graduation it became apparent to me that I had not given enough thought to what I was going to do next . All my friends 7) applied / were applying/ /had applied for jobs in a frenzy and some had had interviews already and were waiting for the results.

I think "were applying" works but I feel like in my opinion I wouldn't use it, but that's mainly my preference. It makes it seem like they never finished applying, which is true because during that time they weren't done after one interview, but I feel like it almost makes it seem like they were applying for a very long time, as if it was a status they carried for a while. Whereas just using "applied/had applied" makes it seem like they didn't take long to get the jobs and didn't keep the "applying for jobs" status for a while. If you use "had applied" you emphasize the amount of time it's been since then, but not by much.

In my defencse I can say that it 8) didn't cross/ hadn't crossed my mind because I had been preparing solidly for the final exams.

I think that "hadn't crossed" is the best since "didn't cross" is used less.