I actually wouldn't allow changing electrical charge, but other than that everything else seems legitimate.

Mages that tweak laws and let reality ensue are just being creative.

The forces mage could cause lightning to strike sand and create fulgurite.
Or they could create heat enough for metal to melt.
Or create a sound that shatters glass due to hitting the right note.
There was a bridge that collapsed to vibrations created by the wind at a particular speed. It was a construction flaw (that modern bridges are engineered to not have)