To answer the first question, I don't think there's any difference in how we may translate "unus tribunorum" vs. "unus ex tribunis" if we understand both to have a partitive meaning. However, Allen and Greenough's Latin Grammar notes that the partitive genitive (which they call "genitive of the whole") indicates the greater group to which the one belongs. In this case, the tribunes. Link here.
On the other hand, Logeion's definitions of "ex/e" suggest that while a partitive definition may be understood, the more general use of "ex/e" to indicate motion might suggest removal from that group. Logeion also suggests in their short definition, "from within". So there's this nuance of inward-to-outward motion. More details here, specifics about the partitive use of "ex/e" at III.B.
As to your question on the difference between "de" and "ex/e", Logeion also has an excellent entry on "de". The short answer here is that "de" can in some contexts mean "from the race/class/status of", so "unus de tribunis" could potentially have the nuance of someone of tribunal rank or from a tribunal family (regardless of his actual rank). Link here.
These are just my conjectures and the nuances/differences I personally get from each. Hope this helps.