Kerry didn’t have any authority to negotiate with the Iranians then, but since when has that ever stopped him. It didn’t stop him from “negotiating” with the Sandinistas under Reagan. The difference is that Reagan told him to take a hike. Obama made him Secretary of State.
Oman as host for secret Iran talks dates back to Secretary Kerry’s days in Senate (including an unusual Senate partnership with the administration), per a senior administration official: “In 2011, [Senate Foreign Relations] Chairman Kerry began a relationship with the Omanis, who had been integral to persuading Iran to release the American hikers. Oman was interested in helping to facilitate dialogue between the U.S. and Iran. He kept the White House fully informed of his contacts with the Omanis and had regular meetings with them about it. On December 8, 2011, he secretly, unannounced went to Muscat [the capital] for a quick trip, missing the cloture vote on Rob Cordray’s nomination, which was never explained to outlets that asked about his whereabouts.
“Kerry returned and briefed the White House. Kerry continued speaking with the Omanis and the White House, and in 2012 a team — including a Kerry aide, along with two State Department officials and a W.H. aide — went to Muscat to begin a U.S./Oman/Iran channel. When Kerry became Secretary, it was decided this channel would continue to help feed into the P5+1 talks, and Kerry visited Oman himself in May.”
Obama no doubt found it convenient to let Kerry conduct his own secret negotiations that he wouldn’t be responsible for, until the whole thing developed to the point where it was a done deal.
This makes it likely that the supposed unofficial contacts by administration people with Hamas were following the same pattern of plausible deniability while exploring real possibilities.