It should be relative to the .dae..
I am saying this because, I tried opening it using FBX Review which renders 3d.
FBX Review loaded the 3d model without the textures.

Moreover, the application does not know anything about textures. It just renders whatever is in the .dae
and all the information about the textures is inside the .dae...

My concern is that if some .dae file is created on Windows which refers textures as in my example, how will I be able to open the same on Mac (with textures), and vice-versa.

I tried changing the path to full path too in the .dae.
No change.

What you see is the "default" light. AFAIK you can get no pointer to the default light to change its position, color, intensity or the like. If you want to have control you will have to add your own light to the root entity which then replaces the "default" light by some internal magic.

So you do something like this:

m_pLight=new Qt3DRender::QPointLight();
Qt3DCore::QEntity *pLightEntity=new Qt3DCore::QEntity(m_pRootEntity);
pLightEntity->addComponent(m_pLight);
m_pLight->setIntensity(m_fLightPower);