If I use Switcheroo on my Mundo and my own Traveling Merchant, does Mundo refresh himself and become 2 + cards in trash?

Ruling: No. Switcheroo snapshots the might difference at resolution and applies continuous modifiers. Mundo will get -X Might and Merchant will get +X Might, where X is (Mundo's might at resolution - 2). If more cards go to trash after resolution, Mundo's might will increase but the Switcheroo modifiers remain at their snapshotted values.

Sequence:

• Switcheroo resolves and calculates X = (Mundo's current might - 2)
• Merchant gets +X Might (snapshotted, continuous)
• Mundo gets -X Might (snapshotted, continuous)
• If more cards go to trash later, Mundo's passive increases his might, but the Switcheroo modifier stays the same

Nuances:

• Switcheroo applies the inverse of the might increase to the unit with more might
• The modifiers are snapshotted at resolution time, not dynamic
• You can use Switcheroo on units controlled by any player