Lets say Frodo has a Song of Kings, can I use Good Meal, to lower the cost of an Leadership event (say Grim Resolve or something)?

Not sure about this one.

You can.

What happens then, when I use Good Meal to reduce the cost of Thicket of Spears? They seem to contradict each other.

Thicket of Spears is essentially non-reducible. In addition to the cost of the event, there is an additional requirement - that you use resources from three heroes. If you can't do that, you can't play the card.

So Good Meal will reduce the cost of the event, but the additional play restriction on Good Meal will then be unable to be met.