It will need to go in the
opposite direction to the normal rotation of that countershaft, to undo it.
Start by giving the hollow end of the nut a good shot of
decent penetrating lubricant, and let it soak in for 24 hours.
I.e. NOT WD-40 - I recommend any of Penetrene, CRC 5-56, Inox/Lanox, of the readily available brands. A home brew of 50/50 Acetone and Automatic Transmission Fluid works pretty well, too
If that inner collar doesn't have set screws, most likely it has an internal key to locate it. In that case, the holes are to take a c-hook pin spanner, to hold the shaft still. While these can be bought, it wouldn't be worth it for one job.
I would find a pin punch that's a neat fit in the hole, and support it at the end nearest the collar with a piece of hard wood, braced against some nearby part of the mower frame, in the same plane as the collar. Wood is easy to cut to the required length.
As for the nut, the through hole will take a steel rod of the appropriate diameter.
Please DO NOT consider using serrated jaw pipe wrenches, or pliers like Vise-Grips, here!
