It's not a single sided swing arm.....so has a spindle going through the middle of the wheel.....so I'd think that it's very unlikely the wheel will fall off...... I'm not sure what that bit is....whether it's a cush drive or not.......but the scraping noise could be.....chain tension too tight?.....did you check the tension at different points on the chain?......disk brake rubbing?.....this is normal, but are you sure there's enough pad material left on the pads?.......wheel bearings?......is there any lateral play in the wheel?......ie, if you grip the wheel with one hand at the top of the wheel and the other hand at the bottom can you rock the wheel side to side? Just a couple of things to consider......