For an 8 M€ card with a building tag, you would spend 4 steel. No matter the worth of your cubes, you would pay that. So you would get a copper cube back for a silver one.
Where you don't get change back is if you paid 6 steel for an 11 M€ card or if you paid 5 titanium for a 13 M€ card.
This is correct. The rule says that you can overspend with steel and titanium, but then surplus value is lost. You can pay 6 steel for an 11 MC card, but in most cases it would be more economic to pay 5 steel and 1 MC instead.
The cubes only represent the number of resources. You can exchange with the bank at any time