I think that we can probably infer from your timeline that the job offer (Navy Sec.) to Sestak was made sometime between early February, 2009 to, at or around, March 26, 2009- contrary to what the White House and Sestak claimed last Friday that the offer was sometime "last summer". Sestak likely turned down the offer and told them he would stay in the race, so then Obama nominated Mabus. Any offer by the White House to Sestak after March 26 would've been predicated upon Mabus stepping down or being replaced -- possible, but less likely.
A possible related note: I have no idea whether this is relevant to your timeline at this point, but it might be, so I'll throw it out there: The White House Counsel at the time the job offer was made to Sestak was Gregory Craig. Robert Bauer, a former Democrat election lawyer, who wrote the Sestak legal memorandum released last Friday, now holds the position.
Rumors started leaking last summer -- less than half a year on the job -- that the highly experienced Yale Law grad Gregory Craig was already on his way out, and were reported in many media outlets (WaPo, WSJ, etc.). On July, 22, 2009 White House Deputy Chief of Staff Jim Messina denied the rumors, "Sounds like typical Washington parlor games to me. These rumors are wrong." (WaPo), and denied it again to the WSJ a month later...
...In September, Craig told a group of three people, which included the president, that he was thinking of resigning (according to Politico). He then announced his resignation in November, with Politico reporting that, "[H]is decision was unrelated to the difficulties in closing Guantanamo Bay prison or other policy issues." Politico also reported that, "Craig said he could 'think of many ways in hindsight that I could have better served the president' but declined to be more specific."
So, who knows Craig's specific reason for resigning, but inquiring minds might want to know...
One more data point: in news you definitely could have missed, White House Deputy Counsel Daniel J. Meltzer resigned earlier this month:
...Among other things, Mr. Meltzer played a leading role in the administration’s efforts to close the military prison at Guantanamo Bay, Cuba, and related policies affecting terrorism detainees.
He was also the White House’s main contact with the Justice Department’s powerful Office of Legal Counsel, which evaluates whether proposed policies would be lawful...
Anyone willing to bet that our beloved investigative pro journalists at the "newspaper of record" will diligently research these matters?