I think he was planning to shoot up, not smoke, the crack. Hence the needle. The lighter was to heat the crack to a liquid in order to shoot it. I assume the belt was to tie around his arm to pop a vein?
On Kevin's addiction story, I think they're getting it mostly right. My ex was an addict, starting with crack and a stint in rehab before I met him. He was sober for six years and then hurt his back and started getting pain pills prescribed. The first time, the addiction took months to progress to the level that Kevin's is at. BUT, the subsequent times (and there were many), it was quick and seamless the way Kevin's has been. And each time, I did not notice (or rather, I did not allow myself to notice) until it was really, really bad. Until you've loved an addict, you'll never fully grasp the denial part of that love. You believe the excuses and you even make the excuses on their behalf when there is no explanation. You hold on to every tiny last bit of hope that they are not using again and that you have not "let it happen" again. Breaking that cycle of denial, excuses, and ultimately enabling the addiction can be just as difficult as kicking the addiction itself. I don't fault Sophie or Kate for not noticing or recognizing (yet) the extent of Kevin's addiction, they are still very much in the denial period. Although, Kevin's breakup should be Sophie's first clue, it may not be since he's hurt her before (although I guess it remains to be seen whether or not Kevin cheated during his previous addiction or after he got clean). But relapse is honestly the very last consideration when you love an addict. You don't want to ever believe it's happening, again.