I suggest you avoid making a screwdriver slot. You can deliver little torque with a screwdriver bit compared with a six point socket, and the slot will weaken the bolt head to a fairly serious extent.
I had thought your bolt head was originally 5/8" A/F, but if you actually had a 9/16" bolt head (14.28 mm) and you were able to put a 14 mm socket on it, the bolt head must be substantially damaged.
The absence of the washer under the bolt head makes the situation worse, because the end of the socket is slightly radiused, further reducing the amount of engagement. If you are prepared to damage the blade plate, you can apply an angle grinder to grind back the blade plate around the bolt head by a full millimetre, then hand file or, if you have one, apply a diamond cutting disk in a small angle grinder, to make a perfect hexagon head out of the mangled mess you currently have. You will probably then need to undercut the blade plate from beneath the new hex head, because initially you'll have cut a hexagon into the blade plate when you cut one onto the bolt head.
Making perfect hex heads is a tricky process, and you need to succeed first time because every time you make it an extra size smaller, you reduce the torque you can apply to it. This is a process I've been through a fair number of times, though not on lawnmowers, and I've had both successes and failures. The failures were always because I didn't achieve a perfect hexagon of the perfect size to fit firmly into the six point impact socket.
And if you finally find you've had too many tries and there isn't enough head left to continue, you just grind the head off the bolt, then grab the bolt-shaft that was formerly inside the blade plate with vice-grips and unscrew it from the PTO. You are only out one bolt, which was pretty much trashed when you started, and one blade plate which is pretty much the price of a ticket out of this mess.
