Correct. On an isolated field terminal neither brush should be in contact with ground.
As far as the 15 ohms through the rotor, its a bit high but measuring through the brushes isn't something I'd put much weight in.
Next measure green wire to ground and blue wire to ground. They should both be open (no continuity).
That's different.
Fusible link melted?
Which way did the ammeter max out?
