Also enabling FSA from the linked scene leaves you with pixelated render, it never does the sample mixing

I tricked blender to allow FSA by adding a camera to the compo scene and adding an unconnected renderlayer into the nodetree. it seems to work when I enable FSA in the compo scene

will check later, but FSA indeed is only working for 3d renders, not for image compos without render layers.

ok, but this is linked scene so there are renderlayers pointing to those linked scenes. The scenes have been set to FSA beforehand


cheers!

I've cleaned the code for checking valid settings on FSA. It also now draws correct statistics in image header, to show which sample is being processed.
Current code survives your test files OK in various versions.

Note however, that FSA is a per-scene (per 'render') setting, allowing to mix renderlayer nodes with or without FSA. In your example the linked scene will therefore be rendered without FSA, and the compositor just uses that image 8 times for every 'full sample' step.

Also when the compo scene itself has FSA disabled, it will ignore FSA in linked scenes altogether.