I'm inclined to think BW is the most credible witness. Not only did she know Sjl pretty well, she was out and about from her own office so was able to verify her timings with them.
But, and it's a pretty big but, for me it simply does not make sense given what we think we already know about sjl's possible motivation for leaving the office. We know she left at between 12.30/40pm that day, with or without keys to a real or concocted viewing. She knew one if the Sturgis bosses was at CT with MG. If the viewing was real, she'd be back in the office within what? 3/4 hour tops? If the viewing was a cover, she'd probably want to be back, for the sake of appearances, by around the same time.
Yet according to BW, she's driving on FPR two hours later, apparently under no duress. What has she been doing for those two hours?