You can remove the wheel rotor to access the dust/splash shield.
You can use regular jobber length drills.

A grade 5 bolt can still be drilled as they are no so much harden but are made of better quality steel.

A decent HSS drill will work but a tungsten colbalt drill will do it without any problems of having to resharpen it.
You must keep steady pressure on the drill to keep the cutting edge engaged. Turning a drill slower helps you feed more per revolution. Nothing dulls a drill faster than to much speed and not enough pressure.
If you lessen the speed, it gives the edge more time to engaged the steel and not glide over it.
The rubbing and idle action dulls an edge. Too much speed can overhead the the two surfaces.

Using oil or a coolant allows this not to happens and increases the production of removing metal. It when you have control of the shearing action and its heat generation you can go like gang busters through metal.

I suggest that you leave the ball joint plate in place UNTIL you use a twist drill the size of the clearance hole for the bolt in that plate. You only want to start a dimple down into the top of the bolt. This will put you on the center of the bolt.

Then pick a drill thats the "tap drill" size or very close to it, to drill out the center heart of the bolt and leave only the threads. Once you drill to the bottom of the bolt you can pick or spin out the helix part of thread left.

Some people have left hand twist drills for the purpose of hopefully grabbing the bolt and turning it out of the hole. Good idea, if you got the bucks to keep those in your too box. I don't, as a screw extractor set, is just as handy for less money. It also takes up less room in the box too.

Usually in both cases, the heat and oils used to drill loosen any corrosion of the thread as it has seeped between the threads own root clearance and removing the center of the bolt removes all the tension on the threads.

As far as leaving a bolt out I would not advise doing that!
It could compromise a margin of safety designed into that location. Even tightness on a platform attachment is a must, for reliable stability.

Think about where that ball joint is and then think about those three bolts, that are smaller, on the top of the strut tower. The top is locked in up there in a pocket by the weight of the car.
No strains can be put up there like those going on below.

Now see what's down there in the way of engineering.
Like the rims for instance. It would be like running around with only four lug nuts on the wheel that has five. Truck wheels have more and even bigger studs and nuts.

The only place you see four lug nuts on a wheel is on an economy car or like items. It's not just because it cheaper to make but because they are usually a lighter car or a slower vehicle that carries less weight. Small, lightweight utility trailers have four!

It's not advised to go around with one less nut on a wheel but many people do and don't even know about it. There are many tire shops that have miss torqued lug nuts or stripped the threads.

Much like the one you found on the ball joint.
What happens if you lose another one on that joint, if you don't fix it?
Do you really want to down grade that setup?

Remember that I said, "platform stability." The spindle only has ONE nut and the bearings own diameters are the platform. Rules are rules! I am not making this stuff up!

Safety, comfort and reliability have always come at a price with many ways to pay!
Your choice?

Phil