Daan, you already answered this, but if I wanted to find a way to view/modify icons.dat and actually am able to add gif files to the collection, would I be able to import them into gif files, would they be imported into Redblade CG Alpha when I start using it?
Redblade CG is capable of importing the icons.dat, so if Redblade 3.5 can read it, Redblade CG should be able to import it. Note that there currently is no official Redblade way to edit icons.dat.
Will all the customizations I've made in the campaign file, prestige classes, races, items, weapons, etc be imported in the new version (nothing invented, all from the books) so that I don't have to do all this work again?
There is an import function in place. However the importer isn't perfected yet and isn't very high on my priority list so at least for the first release of Redblade CG you would have to fix some issues manually.