The destination history lecture talk is given with a slide deck provided by Viking, with freedom to write their own commentary. I agree that lecturers are available onboard.
If you check out the “almost live” thread that UKTog is currently posting on, she’s got the first day’s Daily that lists all the lecturers and their credentials. My cruise had no at-sea days, so we had two guest lecturers. Hers has four scheduled at-sea days, and she has five lecturers, I believe. In case you can’t find it, I’ll post the link.
Crystal, back in service with new owners, has traditionally had good to outstanding lecturers. They have some reasonably priced single staterooms.