Charging with MacBook Air with 240w charge cable?

Yes. For charging, definitely. For communication (not your concern) the answer gets more complicated: Use caution when purchasing USB-C cables or adapters: they are not all alike. - Apple Community

As far as charging goes, Charge your MacBook Air or MacBook Pro - Apple Support explains in the link within it, Identify your Mac power adapter - Apple Support.

Excerpt:

If your Mac uses USB-C to charge, you can charge your Mac laptop with any USB-C power adapter or display. You can safely use a power adapter or display with higher or lower wattage than the adapter included with your Mac.

It's silent on cables, which obviously cannot provide more power than a power adapter is capable of delivering, but they can certainly limit that power. 240W will not be a limiting factor no matter what power adapter you use.

Yes.

Do I plug into laptop and then into a USB adapter into the mains. ?

The connection sequence does not matter, if that's what you're concerned about.