1) No, you should import it back in using OBJ/Accessory.
The only time you need to import things back in as FBX is if they are rigged somehow and you need that rigging. You don't need this for clothing/hair but if you modify the base avatar you definitely need the rig and need to import back in (with the FBX key).
2) Always OBJ accessory, as noted above.
3) Yes, but you can combine things in Daz, so you could put your Genesis figure in an environment, for example, and import the whole thing into CC3 and then into iClone (you can also directly import most stuff now into iClone so this is more or less obsolete).
You can import things into iClone using 3DX -- this is mostly important (and necessary) for any avatar/character/rig that is not CC3 compliant. So a creature not based on Genesis, a truck rigged with doors that open/close, etc. You can directly drag this stuff that isn't an avatar into iClone but I think (can't be sure, Peter or some expert like Rampa would have to comment) you still need to own 3DX for this to work (since I own it I can't say otherwise).
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Mike "ex-genius" Kelley