Hallo,
1) yes, avast for windows is able to catch MacOS X / Linux malware too. The problem is that Windows don't support well foreign filesystems (ext2, ext3, <s>murderfs</s>, oops, reiserfs, xfs ... and macos x's hfs/hfs+), in contrast to all *nix based systems, that usually support wide scale of all thinkable systems (even amiga's ffs/ofs and other cuties).
2) yes, the incremental update is necessary here, and the very next version will use it. it's finished, it works, and now it's being integrated to the gui cover of the whole solution.
regards,
pc