overexposed areas and artefacts

[max.m]

hi, i face a view problems in my actual rendering.

1.) i get too bright areas in shaded regions where the windows hit the walls.

i only can get rid of that by using override materials for the walls and the windows with quite dark gi material (rgb 40/40/40).

by using brighter gi materials the overexposed areas return.

but with the dark gi material i of course get very dark shaded areas...what i want to avoid.

 

is there a way to exclude an object(walls) to send gi to another object(windows)?

 

 

2.) the artefacts i solved by rerendering with lower settings - see the image below. can someone declare me why i get rid of the artefacts by lowering the settings?