I actually doubt that he had any plans to kill him even for quite a while after Simba was born. I think he had planned Simba's death quite a while before Mufasa's. I get the impression he didn't really start to think seriously about killing Mufasa until the elephant graveyard business, realising that he wouldn't be able to have Simba killed with Mufasa around protecting him.

However, I do wonder if, had Mufasa never had any cubs (and Scar therefore keeping his position as next-in-line), Scar might have still eventually become impatient and arranged for his brother to die so that he could take his place quicker.

It’s possible. I never thought about it before.