But why would he be quiet in 2 if he only killed Trish in an alternate timeline? Surely that wouldn’t of had any real effect on the actual timeline that he’s in because Trish is in 5 and that’s set after 2.
I am confused.
Trish's absence from 2's plot suggests she had split off from Dante again at some point or was on another job. I'm guessing he did want to talk to her about it but she wasn't present. Then he's back in his dimension mere seconds before he gets another calling to stop demons from taking over the planet.
In short, you have a person who has just been through a very traumatic experience, has no one to talk to about it and now has to bury his feelings to do his job. It's not too surprising he isn't in a very talkative mood.
I understand the part about it being him to actually “kill his mother’s image” this time around, but really Mundus already messed with Dante’s mind by creating Trish in the first place. And since he already knows she isn’t really actually dead and his mother - no matter what - still is, I just don’t personally think that’s something you can say “Okay this is why Dante is so different in 2”. Dante in his line of work already knows that he’s going to have a lot of difficult and strange obstacles to overcome, especially since he’s dealing with the supernatural. And again, I didn’t really think he was that much different to his DMC 1 self. He just took things more serious than his DMC 3-4 representation.
Intellectually, Dante knows Trish isn’t his mom but on an emotional level, he still associates Trish with Eva because of how she looks.
That said, I do agree with you that he wasn’t all that different. Hell, you could even speculate he was going through some emotional turmoil in 1 due to Trish reminding him of his mom and him going to fight the demon that killed her.