Door questions: threshold, cavity closure and lintel closure

[PatriciaLe_o]

I'm having some trouble with my doors settings... First of all I don't get the threshold the way I want: it doesn't reach the casing limit and doesn't interact well with the slab accessory I'm using (even with regular slab it doesn't interact!) The second problem is with the closures: with the composite walls I'm using I don't get perfect lateral closures and there's no option for lintel closure! I can't believe that the only way to get it is using a special profiled beam as a workaround! Is there somehow I can solve those problems? Is Cadimage Door and Window Builder tool the solution???

[PatriciaLe_o]

another image of my subject...

[PatriciaLe_o]

the last image!