No, the GBuffer is not available for texture sampling in the forward renderer. Look in your material and you’ll find a SceneTexture node. Only SceneColor is still supported.
The forward renderer jams all features into one shader (by definition of forward) so there are fewer texture samplers available for your material. You can usually solve it by using shared samplers.
Yes, any masked materials automatically use OpacityMaskToCoverage. Basically the opacity mask input gets turned into MSAA coverage. You can use it to do dithered transparency with 4 levels for 4x MSAA. It’s very useful for foliage. It was cheap enough that we just enabled it by default instead of making it a material option.
Specular aliasing is an extremely difficult problem, and without Temporal AA it’s definitely way worse. There are some areas for research but they’re all unknowns.
Atmospheric fog isn’t handling MSAA correctly yet (much like dynamic shadows). For height fog we got around this by defaulting to vertex fogging (project setting), which means it is computed in the forward BasePass and therefore anti-aliased correctly. Atmosphere is still computed in a deferred pass off of resolved scene depths so it doesn’t anti-alias correctly. Hopefully something we can fix in the future.