No, spinning the crank backwards with the rear-wheel off the ground doesn't mean a thing. You're not applying any power to drive the bike forwards when you spin the crank backwards, so there's no need to do that while testing or when actually riding.

If you lift up the front-end and spin the tyre by hand and it goes for 3-5 revolutions, it's fine. Same with the back end.

How does spinning the crank by hand feel? Smooth? You may want to remove the chain and spin the crank.

Also did you get a new chain and lube it after the mud ride?