Dunkelzahn death near the Watergate left no body. He edited his Will to bequest the Dragonheart to Ryan Mercury, showing he considered it could end this way, but he nonetheless left the article bequesting his body to the Dunkelzahn Institute.
1. Dunkelzahn returns in the mid to late 2070's, restored by some weird Artifact or other that the Draco Foundation located. What would he do? How would the other GD's react?
It would probably depends a lot on how the other dragons would consider the magic used. If "tainted" in any way, Dunkelzahn would be an outcast, and would thus likely keep a low profile rather than face an alliance against him. Either way, Dunkelzahn may not be the first dragon to pull such a trick: Dunkelzahn may be the most powerful dragon of the modern era, he is not the only to ever have lived and died.
Ghostwalker sets at least one sort of precedent we know of. Even if it's not clear if he was considered as practically dead or simply "on vacation", none of the other dragons denied him his status, and they even acepted is demand of a replay for Dunkelzahn Rite of Succession.
2. Dunkelzahn's death was faked, and he has been behind the Draco Foundation all along. What has he been doing all this time?
You give the answer to your own question here: he did lead the Draco Foundation all along. Except maybe for some behind-the-scene magic stuff, the Draco Foundation wouldn have had everything Dunkelzahn wanted done.
In this scenario, it's quite likely Nadja Daviar simply hide with her master between 2064 and 2073.
3. Dunkelzahn's death never happened. He either survived the explosion, or the explosion never happened. What would an active, public Dunkelzahn with the might of the UCAS behind him have done? How might the 2070's look differeng if this occurred?
Kyle Haeffner retained an unprecedentely high popularity at the end of his second term, and there is no limit to the number of term a president may serve in the UCAS. Had Dunkelzahn enjoyed similar levels of popularity, there would probably have been a growing fear among political opponents of a dragon ruling UCAS for millenias to come. I can see him possibly serving a third term (as there are human precedents for that), but too many people would feel uncomfortable with a dragon becoming the first president of the union to serve four terms.
On the corporate scene, there would have been no war between Ares and Cross, at least as long as Dunkelzahn forbid Damien Knight to move against Lucien Cross. I think Fuchi break-up was unavoidable. Villiers, Nakatomi and Yamana already were at each others' throat prior to Dunkelzahn's death, and Renraku already had Brightlight works underway to encroach on Fuchi market shares.
Without the Watergate Rift, Ghostwalker and shedim may have been unable to return. If Ghostwalker nonetheless returned through another mana anomaly (Crater Lake?), there could be a heated argument between the two brothers. Afterall, Dunkelzahn let Aztechnology controls a part of Denver area, a place Ghostwalker vowed to protect.