Why would Congress pass legislation saying that it's not allowed to amend trade agreements? Why don't they just not amend trade agreements?

Because it makes it very easy for individual members to hold their hands up and say "there was nothing I could do" about things they would like to pass but know might prove unpopular. And it minimizes the window for the public to mobilize opposition.

We're talking about the several-hundred-member body of people whose very job is to argue with each other? The one whose membership is up for modification every few years? The one whose purpose it is to create rules that persist across changes in membership?

That Congress?

But the new Congress can just as easily pass legislation to invalidate the previous legislation.

Yes, just like judicial precedent doesn't exist whatsoever because any judge can overturn any decision when given the opportunity.

Also, we rewrite the Constitution because Amendments allow us to. It's just that we do it so fast and then change it back that you don't even notice.

Congress now != congress in 2 years, remember.

Yes, Congress now is only ~90% of what congress will be in 2 years.

its hard for me to think of an answer to this question that doesn't somehow devolve into an area of 'conspiracy'-type BS.

I really do not know.

Why are alternatives to the current mainstream stories have to be 'devolving' into anything?

In this situation its probably a matter of 'don't attribute to malice what you can attribute to stupidity'?

OTOH - the mainstream media is not reporting on this at all. Doing a search for 'trans pacific partnership' on CNN - the first result is a story about the G-20 Summit on Jul. 15th.

The more you look at, investigate and try and understand this story the crazier you may seem to other people.