So, seeing as how a C'tan with Writhing Worldscapes stacks with Orikans rule on the first turn to make all of the table at least count as difficult when you move, and the C'tan then makes that dangerous, would a tank that move count as moving through difficult terrain only, therefore dangerous, and then applying the C'tan's rule and making him fail on a one and a two, or would the tank just count as moving through difficult and dangerous terrain as normal?

Yes.

Same applies to the Tremor Stave.

Yes to tanks take dangerous dangerous terrain? or they don't?

Immobilized on a one or two

Brutal

For nearly 500 points, hardly brutal.

Plus a dozer blade or siege shield allows you a re-roll/ignore completely the DTs... 500 pt ability countered for what? Max 120pts or so?

Or you could just not move

unless it's DoW and you're not in reserve , but yeah not moving is fine too.