I respectfully disagree. I don't think his executive assistant would be traveling with him to this. They typically would work out of the home office (in MN).
I also disagree that his schedule would be easy to find OR well publicized. It's possible they might be able to find a meeting agenda to show what time he was speaking that day, but not detailed info if he flew in the night before or morning of or what hotel he was staying in.
If I went online and tried to find the schedules of Fortune 500 CEOs I'm pretty sure I would come up empty handed, or it would be very, very high level, not enough to plan a murder on.
Which leaves us with the option that someone "hacked" into BT or his assistant's devices, which I just think is unlikely. I cannot think of another websleuth murder where that every happened - it sounds like something out a CIA spy movie

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Which then leads me back to my original theory, someone who knew his personal schedule, was in contact with the shooter - either someone he worked with, socialized with or had his phone on their Life 360 plan. It's just the simplest explanation.