Any links to an OM so one can see your actual schematic?

IF your system has an Operator Presence Relay, disconnect that and see what happens.

Usually, "jumping" the seat switch guarantees no spark.
You are merely grounding the kill wire.
Seat switch likely has to be OPEN to allow spark.
Simply unplugging it doesn't work either because of the shorting connector.

Having the actual schematic is really important to accurate diagnosis. otherwise it's a matter of WAG's.

You can disconnect the kill wire from the magneto (under the flywheel cover) and see if the engine runs or sparks with the wire disconnected.
If the engine sparks or runs with the wire off, your problem lies in some part of the chassis wiring (and must still be diagnosed).
If, however, you still get no spark with the kill wire unplugged at the magneto, the magneto is defective.
The blown fuse is not related to a problem with the magneto.....the magneto is its own little electric plant for making ignition sparks and is in no way connected to the other electrical system, nor can a magneto cause a blown fuse.