Yeah, agreed. Looks like it wouldn't take much to make it make it twist and fall over sideways. I'd at least put some plywood along the back, that would reinforce it hugely.
No, and all "Old tank syndrome" is, is a euphemism for a tank with massive amounts of rotting crap built up because of poor maintenance. In a tiny, easy to clean lfs tank with nothing but a few mm of gravel I doubt that's an issue.
And yet, all the ingredients are used in human medication. From your description the only possible use for it would be murdering babies. I think you're overreacting and you had an unusual sensitivity to it. Allergic reaction to any of the ingredients is rare but not unknown.
Don't know the exact measurements but that sounds about right, they're about average size. If you can't lift 7kg you probably won't be able to handle any canister filter.
You bought driftwood? Why?
Just throw it in the tank and weigh it down until it sinks. You can go through a bunch of work boiling it to make it sink, but there's no point, just throw it in the tank and put a rock on it.
If you're worried about the anti-caking agent just check the label, it should tell you. Most common(I believe) is 551 which is Silicon dioxide. Which, if that's going to hurt fish they're in BIG trouble...
$100...You might be able to beat that shopping around, but it doesn't sound like a bad price if you take into account not having to buy individual bits. And I'm guessing you probably don't have a good idea what you want/need already.