The advice I always give is its a gamble, sort of like buying insurance. If you spend time and money chipping into that rock and then pouring concrete, you can be reasonably sure that it won't fall over.
If you stop at 1.5 feet and use a mud slurry, you dramatically increases the chance that a freak wind storm, impact from a lawn tractor or a fat kid trying to catch an overthrown football is going to knock it over. But then again, you may luck out and it stays up right. (I would just make sure that if it falls, it doesn't take out anything expensive, like the house.)
If I were you, I would probably spend 15 minutes working on the rock. If you are getting nowhere, clean it off and use some cement to hopefully encase part of the rock and the pole.
Also, considering leaving the extra height in there. The taller the pole, the better your production is going to be. There is a limit, but that's somewhere around 16' above ground.