Possible blown head gasket or worse

Well, you are working fast! Any brown foamy stuff in the oil? Look in the PCV and pull the valve cover off and look under there. If any water/coolant had been in the oil for any length of time, you will usually find some brownish foamy gunk (stuff that looks like it was floating on the top of a septic pond). With the valve cover off, looks to see if all the valves are in place and no springs are broken.

Well, you won't get valid compression readings with the enigne not spinning freely. Did you take out all the plugs and see if it spins better? Going back to your original description, it sounds like maybe the starter is not engaging the flywheel right. Is the starter tight?

If the starter does not work consistently, do this to see if the engine bottom end turns freely and check other possibel problems at the same time:
- Take out all the plugs to relieve the compression pressure
- Pull up on the fan belt pretty hard between alternator and crank pullies, and turn the engine by hand with the fan as a lever. Better yet, get a 3/4" x 16 thread, 1-1/2" or 2" long bolt and wide flat washer, clean out the threads on the inside of the crank snout (in the hole in the crank pulley/dampener), and put the bolt in, and turn it with a wrench (1-1/8" across the flats for a 3/4" bolt)
- See if the engine turns consistently freely with no clanks or anything tightening up' don't force it. Also, have the valve cover off and see if the valves move up and down completely; if not moving at all, the timing chain is broken. Look at each valve carefully as it opens and make sure it smoothly starts to close right after it stops opening as you roate the engine, and then close ALL the way. Check all of the pushrods to see if any are bent. (See below)
- While you do this, take out the starter and slowly rotate the engine bit by bit and look at the teeth on the ring gear to see if there are any stripped spots. And look at the teeth on the starter bendix gear to see if they are chipped.

Aftrer this if the crank turns freely, I would try to run a compression test as that is good info to have...... assuming ALL of the valves are free (see below). Put some oil back in first. If the crank is free, then it perhaps is a bad starter or the ring gear is shot. Is this an auto or manual trannie?

Also, has this car sat for a while? If so, the varnish in the old gas can cause the valves to stick and it will cause all sorts of bad running symptoms, like a bad cylinder head gasket. This is surpisingly common. That is why you should have the valve cover out and carefully observe the action of each and every valve while rotating the engine. If some are sticking badly enough, they could be being hit by the piston and some may even be bent; any bent push rods are a sure sign of this going on.

If you suspect any sticking valves, pull off the rocker assembly and tap each valve open with a hammer and a hard wooden dowel (or a hard plastic hammer), and see if each valve snaps shut. See if any of them show signs of sticking.

Merry Christmas!