newbie question: trample ability

[Groundworm]

So I understand how trampling works in its purest form. The elephant stomps the infantry. And the cavalry. And the troglodyte. Got it.

But what about magically enhanced units? My specific questions are:

1) how does high defense stop trampling, if at all? if I have a unit with 20+ defense going up against one trampler, does his defense get whittled down with each trample?
2) do trampling units get a number of trample attempts proportional to its AP?
3) how does trample work with ethereal units? mistform? luck?
4) does protection negate damage taken from trample?

These questions assume a thug or SC that is significantly smaller than the trampler.

Thanks in advance,
Groundworm