Not turning over - 2.5V6

[Skellibert]

G'day folks.
As you may know (or may not!) I have a 2000 Mondeo ST24.
Several weeks back I drove into our driveway with absolutely no problem nor any indication of anything amiss.

When I got into it the next day, the starter would click (solenoid throwing OK) but would not spin.

When I looked under the bonnet, the main starter cable was shorting on the battery bracket, and had burned some of the plastic insulation from that cable. The Cable itself appears entirely intact, and I taped it with electrical tape to make sure it was not shorting. The plan was to get it going and buy a replacement cable.
But it won't turn over at all.
After fluffing around with various bits and pieces, removing the starter, and finally replacing the starter, I am getting the same thing.
Solenoid throws, but no spinning. However, the new starter definitely has more grunt behind it. It feels like it is trying to spin.
Anyway, I can't seem to get the motor to turn over on the starter, and I am beginning to think that it is a mechanical issue - some sort of seizing of the engine itself.
Today I pulled the three plugs from the front bank, jacked up the driver's wheel, and tried to roll the motor from the tyre - could not get it to turn.
Is it reasonable that there would be enough compression from the back three cylinders to make turning the motor that way difficult?

Now I am at the limit of my knowledge, and so I am asking here - what is the next thing to check on the little V6 to see if the motor is broken internally? (Without pulling it apart totally!)

Remember that it drove into the driveway without any issues at all - this developed overnight.
And the coolant is clean, and the oil is clean, no indications of a leak from one to the other. (I wondered if it was hydro'd from a cylinder leak - not the front three at least!)

I haven't pulled the rear three plugs because the coil etc is bolted over the top of it, and up against the firewall. But the front three are all dry and clean - I don't have a scope, but I could see inside with a torch (just). And the plugs that came out are old, but clean and dry.

Any help is greatly appreciated.