Is it safe to mount oil cooler in this position?

 

Also, Is it safe to leave the stock speedometer mounting hole unplugged?

 

You need to mount your oil cooler where there is less chance of getting road debris or a stone through it which would REALLY spoil your day. Careful not to interfere with the suspension/fork travel, and fit a steel mesh grill in front of it.
Fill the speedo hole with silicon - keeps the dirt out and easy to remove if you need to.
Hope this helps.