Yes, that makes sense on the diode failure. Both dealers have the diode heat sink assemblies, I believe. It tripped your house breaker because the batteries could not "pull" current through the tractor circuit breaker with the bad diode.
Easiest way to access the charger is to remove the wires on the charger cover (with the disconnect off, of course) and remove the screw holding it to the grill. Pull the cover up and disconnect the charger timer connector, then remove completely. Two bolts hold the charger to the frame. You'll need a 1/2" socket with a long extension for inside and a 1/2" wrench underneath. Once the bolts are out, you might remove the closest battery, so you can flip the charger up on its' side and disconnect the positive wire on the heat sink (usually orange insulation.) The ground wire will usually come loose without holding the bolt from underneath the charger base. Once both wires and bolts are disconnected, you can lift the charger out.
Without the wire connected, you were getting "half" of the charging power, so it would have taken twice as long to charge. Either way, it needs repaired and would never come up to full charge.