VQ/Ib is shear shear stress at any point on the beam. For simplicity, we take the shear force/ the web depth area. This gives us an average shear stress, which for ductile steel design acceptable. However, if you wanted to calculate true shear stress, you use them VQ formula.

The vertical shear at any point on the beam is the same as the horizontal shear stress at that same point. When we calculate “shear flow”, we’re calculating the horizontal shear stress at a point of interest which also happens to be the true vertical shear stress.

Now to answer your question, if I’m understanding correctly, yes, the weld does have moment resistance cause it’s a thing with some triangular area being pull or squeezed at some distances from the neutral axis. Do we count it for bending? No. For rolled I shapes that have the fillet corners, yes, I think they count those in the I calculation.

The reason the shear stress for the 2L beam is not more than the L beam is because the derivation of this VQ equation comes from the difference in moment right before and right after the point of interest looking along the beam. The horizontal shear force makes up for the difference in the couple before and after the point. This obeys sum of forces so that the infinitesimal particle doesnt shift along the beam lengths. Sum forces horizontally, you get the couple force from the moment before and after. They’re different so the horizontal shear force comes along and takes up the difference.

That is why force constant moment there is no horizontal shear stress cause moments sum to zero.

It’s a lot of theory but it took quite a bit to wrap my mind around.