The engine cranks and the starter flops around when it's loose on the engine. That tells me there's a mechanical bind, not an electrical problem. I've suggested putting a couple of wheel alignment shims between the starter and the bellhousing, bolting things up tight, and trying to crank the engine. I believe the "no crank" condition is caused by something not fitting right, not necessarily an open circuit somewhere. If the battery voltage drops way down,. like 4 volts or less when attempting to crank a locked engine, that's not necessarily a bad battery- - - -just too much electrical load. Do the battery cables get hot to the touch after a cranking attempt?
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!