You are over complicating and overthinking things. Adding meaning where there is none. Mundus created her and he even offered to create as many as Dante wanted. There is not previous context. She know what she know for the same reason she can speak and fight and knows how to ride a bike, because she was created with that knowledge. Except for that last part I said this is very common knowledge in the cannon.
Here's an official source, just in case:
Excerpt from the Trinity of Fates book:
D E V I L -M A Y - C R Y - 1 - : - C O L U M N
Existing in the heart forever, the pure love of a mother
- Trish was made by Mundus as a living copy of Dante's mother, as bait to lure Dante out. When the two of them met, there was a gruesome battle, covered with the spray of blood. Mundus' plan worked wonderfully; when Dante saw that Trish had his mother's face he started to trust her.
Well, Mundus didn't kill Sparda and he killed his family for vengeance. Sparda's cause of death is probably never mentioned because it's irrelevant; it's not how he died but that he died.
Then there'd be more solid evidence and Trish wouldn't have to go out of her way to tell V she's not his mommy.
Sparda awoke to justice and didn't want Mundus ruling over the human world. Again, this is all very basic stuff.
What? Have you not heard of clone or, in this case, homunculi? It's fiction. Horror fiction. You don't have to make anything the old fashion way if you don't want to. To make a demon that looks like this other person out of thin air and make her any age you want all it takes is a 'why not?' It's not even that hard to conceive.