Isn’t it supposed to be loose? It’s the same diameter as the first one right? Once you screw the battery tube on it will be held firmly in place. Or you can add a dab of glue.

I would not worry about the differences. The FET should still be fine, the company that makes the driver probably bought a new batch of FETs which have a different number on them.

Are you measuring the wire or the insulation? Insulation thickness can vary, but it has no effect on the performance. If the wire is thinner gauge it can add a little more resistance. This can be good or bad depending on what you want. Thinner wire can slightly reduce the amp draw on the LED while a thicker wire can slightly increase the amp draw.