There's three buttons in the VSE header that let you turn on/off the preview and sequencer:
Do these have a keyboard shortcut? And if no, is it possible to set one up?
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As pointed out by Sambler, CtrlTab already cycles through the three sequencer views.
It's also possible to set up a shortcut to toggle between two of them:
Input, and expand
Sequencer, then expand
Add Newand expand the new shortcut
You could have figured out the last two steps by changing the sequencer view and looking at the output in the Info window.