I bet its not up to code but i dont think its unsafe...
Code here says you need a staple to hold the wire up every 4ft or less. Its not "unsafe" at all unless you run through your basement like a football player an tackle all your gear every day. So yes , common sense says its unsafe but your the one working down there , its fine. Just dont tie metal cords around your neck attached to a lightning rod thats sharp enough to cut through the romex and you'll be fine.
Im guilty too but for fucks sake we are not allowed to hang the light from the wiring ! The weight will pull the wire down a tiny bit at a time and start tearing the romex off at the staple. No big deal just slap some staples on it so its not hanging down. its fine and safe.
Now about the wire your sparky told you is not rated for the numbers but your load will not hurt it...... Thats something you do NOT want to leave that way when you move. ONLY you and that sparky know what that wire can really handle so try your best to get rid of that before moving or selling the home.
Some jackass will come right behind you and use the full 100 amps and burn himself to the ground...... we dont need that on our shoulders.