Is this really need? Isn't is kinda implicit that if all points on one triangle is on the same side of an other, you only need to make the check once?

If triangle A is all on the positive side of triangle B then triangle B must not have any points that crosses the side barrier as well.

It is very vague what this does from just looking at the implementation here.

Perhaps add some comments in the code explaining what this is?

I doesn't seem to be some edge length as it can be zero on a positive intersection result.

Should probably be a comment here explaining that we are trying to find the two middle points in the intersection plane.

A--B--C--D

We want to find B-C as it will be the cross section where the triangles intersect.
However there has to be a change in which triangle the point belongs to.

So for example if the first two points belong to triangle A, there is no intersection happening.

I agree that it is still good, I have changed this name several times and I cannot find a good name to describe what this value represents.
Basically it indicates how many edges of triangle A have been intersected.
For example, if a vertex of triangle B crosses the middle of triangle A, this value is zero because only the edges of triangle B intersect.

The description is a little confusing (I will change).
This actually tests the side of the points against the plane of the other triangle.
Each triangle must intersect the plane of the other triangle so that it is possible to have an intersection.
I think a better description would be:
"All vertices of the first triangle are positioned on the same side to the plane defined by the second triangle".

This is a good way to describe that part. (I didn't think that way).
Perhaps that part of the code can be further simplified.

Ah, right. I was wrong.
There is some cases where there is a sign change on one triangle, but not the other.

So these checks are correct. I do like your suggested updated description!
It is saying the same thing but in a more verbose way. :)

Ah I see.

Perhaps the name could be r_tri_a_edge_isect_count?
Then have a comment further explaining that it is the number of edges that intersect from tri_a.

Change _len to _count

Length is not used for how many times something occurs.

Am I missing something or is this return value never set if there is any intersection?

The return true above makes this code never execute.

Perhaps be a bit more verbose here. So:

/* The triangles intersect because they overlap on the intersection line.
 * Now identify the two points of intersection that are in the middle to get the actual intersection between the triangles.
 * (B--C from A--B--C--D) */

We are rearranging the order, that is true, however the I feel that it should explain that the rearrangement makes it so that the alone vertex is at index 1.

So perhaps something like:

/* Rearrange the triangle so that the vertex that is alone on one side
  * of the plane is located at index 1. */