1) dfonts are a variant of the old resource file (except the resources are stored in the data fork rather than the resource fork). They are mac specific. They contain a resource table of contents and then a bunch of resources (in a dfont, the resources are always fonts, but that isn't a requirement of the format). They tend to be a collection of styles for one family, but again this is not required. Lots of dfonts to be found on any mac. FontForge will create dfonts for you (Generate Family).
2) TTC files are not supported by apple (or weren't) but are by MS. They allow for glyph sharing but do not require it. Most I have seen have a core set of CJK glyphs common to all the fonts in the TTC with several different sets of latin variants. There is no reason why you couldn't have a ttc with a bunch of styles, but I've never seen it done. According to the OpenType standard (http://www.microsoft.com/typography/otspec/otff.htm) "The CFF rasterizer does not currently support TTC files." FF will not create ttc files, but will read them. There is a script floating around that will create ttc files from ttf (or something like that) check the fontforge mailing list archives if you are interested in doing that.
