I will try to answer a few of those right now as good as I can (slowdive will have the other and/or more thorough answers):
1. I will pass this one on to slowdive.
2. I can only tell you that I run it successfully on Win7 and - as it is directx9 based - would expect it to run all the listed OS (but don't know positively). Max. res. of 1024 x 600 should be no problem, using intel integrated weak graphic cards might become a problem (my old netbook can run the toolset, but not the engine right now).
3. Yes - Hearkenwold, Lanterna and Red Carnival are the names of the three adventures available already (see here http://www.iceblinkengine.com/forums/viewforum.php?f=22
). Hearkenwold and Lanterna form each the beginning of a larger adventure, while Red Carnival is (somewhat) closed in itself (but plays in a gameworld I will do a much larger campaign in). You will need the IceBlink engine to play these modules though.
4. The extent of customization is immense. Besides own character portraits, map graphics (which can be tiles or full+ screen pictures), tokens/sprites and icon graphics, you can also change parts of the GUI like fonts or various menu background and header colors. Sounds and music can be self recorded/imported and used in IB (.wav, .mp3, etc.). Furthermore IB has plenty of script hooks (C# scripts) and customization wizards in place: design your own races, classes, skills, traits and spells,. Set up you own shops and modify the rule system via many open scripts the game engine fuses , too. Have labels for gold/credits/Yen or specifically define the content of the character sheet... this and much more (do creatures, items, props... all with in-toolset menus and imported graphics).