I think it looks great so far. The timeline looks perfect, and your outline is nice and linear and doesn't look messy to me at all. Regarding difficulties and clues:

- Do the police start investigating at some point? If so, do they scold Sam and tell her to let the authorities do the investigating?

- It's 2016. Is there an announcement on social media to friends and family?

--- Would people say when/where they last saw him, providing leads for Sam to follow up on? Last place he went to lunch? Last time his car was seen? Things that ultimately waste Sam's time and/or provides clues?

--- Would a busybody stick their nose in and do something that gets in the way?

--- Does Sam's voice mailbox fill up with concerned friends and family, such that she misses an important phone call with legitimate information?

--- Does the media get involved and follow Sam wherever she goes?

--- Does one of the girlfriend's connections private message Sam with some clue that eventually points her to the girlfriend?

--- Could you use flashbacks (real or imagined by Sam) to give clues? For example, say Sam's learned he was seen (1) at the construction site at 10am, and (2) at lunch at Laraby's at about 11:30. Could she visualize the scenes in her head, imagine him driving or walking to lunch? Visualize the route he might have taken? Maybe even trace his steps? Or... you could include real flashbacks, giving the reader a hint of what actually happened, something hidden from Sam, but not enough to give the whole thing away. Maybe even enough to lead the reader astray, to make them think it was the business partner or someone else.

--- Does the girlfriend plant a red herring, something that leads Sam away from the girlfriend? Like maybe make up a story, telling the police she saw a burgundy minivan that looked suspicious? Lie and say Sam used to have a drug addiction? Tell police he owed money to a loan shark? When Sam tries to investigate these things, she could get herself into real trouble, like kidnapped by a drug dealer or loan shark.

Depends on how action-packed and dramatic you want to be, I suppose.

Cheers,
Michelle