Are there any credible conspiracy theories?

There's a 99.99% chance the US government doesn't even know who piggybacked this attack. There's no doubt people demolished WTC7.

NIST hasn't seen anything like this before in a building of this type. It took seven years for them to figure out why the building collapsed due to fire, then the reason turned out to be controversial. Then people noticed their results involving cutting corners and removing construction elements so the building would fail in the finite element model. 

Since NIST denied freefall was possible here when they presented their draft report of the WTC seven collapse to the public, I'm 100 percent sure the truthers have a reliable hypothesis. NIST let people with reliable credentials ask questions about the collapse at the conference. A Physic Teacher with reliable credentials got past the setup and asked why freefall wasn't in the report.

Because freefall happened here, it couldn't be the falling building that crushed the lower half. 

The answer NIST gave to David Chandler exposed that to people paying attention. There's a video of the whole thing, so nobody can claim NIST words were taken out of context.

An exchange between NIST's Shyam Sunder ( Chief Engineer of the report) and David Chandler. 

In Shyam Sunder's explanation, it matches his computer models, so people know freefall wasn't caused by fire. 

His answer exposed everything. Watch the video, nothing's taken out of context.

Quote NIST 

 A] free-fall time would be an object that has no structural components below it.... What the analysis shows...is that same time it took for the structural model to come down...is 5.4 seconds. It’s about 1.5 seconds, or roughly 40 percent, more time for that free fall to happen. And that is not at all unusual because there was structural resistance that was provided in this particular case.”

Please look at the screenshot I showed you there. It matches what he said. If there's structural resistance slowing the fall, freefall is impossible. In the area where the freefall actually happened, the steel was still bending and crumbling over 2 seconds after the building collapsed. For those who pay attention, this reveals everything. 

Your last two sentences perfectly sums up conspiracy theorists.

Almost every engineering firm in the world knows this was the first time a tall steel-framed building collapsed just from fire. Having it happen during 9/11 says something's not right about the story. After that, you got the official US government engineering report, which changed the design of the building itself, so it collapsed. It would never be tolerated to take an engineering course at a university and start removing construction fittings to make a building collapse. Here's what happened, it's clearly fraud to do, but NIST somehow got away with it because of the nature of the event. 

The idea that buildings fall down is simplistic. By the fact that they had seven years of experts looking at this, none of them noticed the building collapsed at freefall until the time a truther exposed them.  

This NIST statement is them denying freefall Aug 2008 ( draft paper release)

A free-fall time would be an object that has no structural components below it.... What the analysis shows...is that same time it took for the structural model to come down...is 5.4 seconds. It’s about 1.5 seconds, or roughly 40 percent, more time for that free fall to happen. And that is not at all unusual because there was structural resistance that was provided in this particular case.”

Break it down

"A free-fall time would be an object that has no structural components below it"

The building had to collapse without structural steel or concrete underneath for freefall to be true!!!

Debunk by NIST.

"What the analysis shows...is that same time it took for the structural model to come down...is 5.4 seconds. It’s about 1.5 seconds, or roughly 40 percent, more time for that free fall to happen"

As far as NIST is concerned, the longer time Meant Freefall couldn't happen. Freefall would be 3.9 seconds Slower time was expected because there was structural steel resistance slowing the fall and the collapse was not instant ( logically mean controlled demolition)

Here's the problem with NIST's freefall theory. In the model, the steel frame was still buckling underneath when the building began to collapse, but the time is off since this is two seconds into the collapse.  

As soon as the roofline started falling, freefall would have happened in 1 to 2 seconds across the whole building. No matter how damaged, concrete or steel still requires energy to break up. Just the fact that a building fell through floors over 100 feet with no resistance says something's wrong. Floors of concrete and steel dont just magically disappear.