Soft mud underneath and very little traffic or wash, very sheltered slip.
Not a problem unless the water goes down so far she's litterally hanging by her cleats. That
would be cause for concern.
As for resting on the keel: No problem. Consider: We just hauled 32 boats yesterday. All of the boats on jackstands and many of those in cradles sit on their keels. The pads on the sides are just to keep her from tipping over
One of the books we read on sailing showed a method for doing bottom repairs w/o hauling that involved hooking up to a spot with a seawall, deploying bumpers on the seawall side, paying-out line as the tide went out, keeping the boat just far enough off the seawall so she'd reliably lean up against it once her keel was grounded, and wait for the tide to run out until the bottom was exposed.