That is probably the likeliest scenario. Reading about spousal murders, it seems to take from 2-6 months between the decision to kill and the killing itself. Planning takes time.
What is throwing me off in this case, I believe, is the length of time Howard watched Anna self-destruct. My instinct is that he finally viewed her life as having no value.
I don't think he planned Daniel's death - I believe Daniel confronted him as being a fraud - blood sucker - enabler -opportunist and was killed as a result.
Anna's fate was sealed when she realized HK$ killed Daniel. She was dead within 6 months from plan to execution.
I'm not so convinced that it wasn't premeditated. Yes, Daniel probably did P him off in the car, but remember the fact that Daniel was going to take over as executor of the estate when he turned 21. Of course, maybe he could've done it at the house at a later time but that would make it look more suspicious.
Anna's fate was sealed when she realized HK$ killed Daniel. She was dead within 6 months from plan to execution.
AbsoANNAlutely.