1)
The speaker is going to say another thing they should do before visiting the place. Does 買っていくのはいいとしても、、、 make sense?
2)
I don't think 得られる is actually used as the passive form.
3)
It can be considered a polite form of ~なくて.
何のお構いもしなくて --> 何のお構いもしませんで
Strictly speaking, this で is more likely a particle rather than a conjugation suffix, though.
4)
And the interpritation "stay and go (home/to the next place)" is also possible.
5)
As Glenn-san guessed correctly, ひょうす has a nuance of written words or formal words. Unlike あらわす, ひょうす is mostly used when people who express something is the subject, and not when the thing is the subject. For instance, in the sentence この紋章は王者の位を表す, 表す is あらわす and not ひょうす. Furthermore, there are some expressions that ひょうす is preferred as a collocation, like 敬意を表す. (And when it's 表する, the reading is always ひょうする, of course.)
6)
Grammatically, the both interpritations; (世の中が)物騒だから(since the world is dangerous) and そうしないと危ないから (since it's dangerous if you don't lock it) are possible. But 不用心 can't be used as the same usage as 物騒, thus, 世の中が無用心 is not used. The subject of 無用心 must be a situation like 夜道の一人歩き, 留守がち or person. I think it means そうしないと無用心なので here, as eeky-san interpreted.