All clutches can go right to the floor. I suppose you mean it takes all the way to the floor to disengage? If so, it needs adjusted. If it's in neutral it should not have any affect on this problem. You also can turn motor by hand. It ain't the clutch. Check your oil. Try it without plugs as said. All information will add up. This needs to be narrowed down. Need to make sure there is nothing wrong with motor. Probably not, now that you say it started at lumber yard and you drove home.
Slightly confusing to diagnose:
1. Good voltage test, I guess. Jerry can decide.
2. Spins good without engaging starter
3. Spins AND starts with jump
5. You say you cleaned all connections and cables are good
6. Battery is new
Jump start was a success, most likely reason it was, is your battery is bad, Jump start could be overcoming other issues but batteries are so screwy. They show good voltage and all that. They break down but still read good. HAPPENS ALL THE TIME Needs a genuine load test with good equipment and operator. FLAPS should be OK. Sooner than later. Take it back where you bought it. Also time to be thinking 12V conversion.
We usually don't tell others to start replacing this and that. We try to solve by logic, testing, troubleshooting, adjustment, settings and past experience. It is, however, a good thing you found a starter. If the donor starter does the same thing it will be a 92% chance your starter was OK. 8% chance the donor starter is also bad. If it does do the same thing it will point elsewhere. Elimination by replacement is a method that we all have to resort to from time to time especially if the parts are cheap.
So keep the faith baby. Your good effort, understanding, doing what is recommended and good reporting is an example of what we wish everybody did.
I still don't like the looks of your cables. But I can't tell from my house.
