𝕿𝖍𝖊 𝕰𝖓𝖑𝖎𝖌𝖍𝖙𝖊𝖓𝖊𝖉 𝕺𝖓𝖊
Hard to say. Lucifero has definitely been portrayed as the strongest demon, yet we've also been told multiple times that the Underworld has three Rulers. Based on this, Lucifero should be above them, but not by an overwhelming amount. The other two should be capable of somewhat competing with him and threaten him, otherwise there'd only be one Ruler the way I see it.
B. He might have been capable of defeating him, but he wasn't sure at all of suceeding. AKA, they were on very similar levels beforehand and couldn't afford to take any major risk for his plan.
I think best case scenario for Lucius, they were on similar levels before these events, but him being beneath Full Power Lucifero is also very possible.
We kinda don't know how strong Lucius could've been on his own prior to having any devil power, but it's very likely he was insignificant to any of the Three Rulers of the Underworld. That is, his own strength added to Astaroth's wouldn't have made much of a difference compared to the strength Astaroth had on his own (assuming Lucius has Astaroth's full power), most likely.