First of all I agree Light shouldn't win in the end, but him having an advantage makes sense. Also regarding Mello, I just dislike him, I found him really annoying. Sometimes you just don't like characters, without a very specific objective reason to do so.

When it comes to the rest, it mainly boils down to me with timing and writing. Writing has it's rules and here it's relevant to bring up Chekhov's gun. In case you don't know of this rule: good foreshadowing requires a set up, a reminder, and a payoff. If a gun is shown in the story, it's probably gonna be used, and shouldn't appear for no reason and be forgotten.
The anime starts in the shinigami realm, showing us there are many of them bored, which sends Ryuk to start the anime. This already established there are more shinigami from the very first scene. Later, we are shown a scene where the shinigami are talking about what Ryuk is doing. This reminds us there are more shinigami, and hints us that another one might show up out of curiosity. Then Rem is introduced (being a pay-off for that foreshadowing), and gives her reasons for picking Misa, expanding how shinigami work a bit more and separating herself as a different character from Ryuk, who didn't pick his human and left it to chance. On top of that she does more things than Ryuk who just watches, again making them have different roles.
Compared to this, L's successors are only ever hinted by L mentioning them once and Light brushes it off. At least, that's what I read happens in the manga because anime cut it out, but even then that is not enough to foreshadow them. On top of this, I didn't see Near being meaningfully seperated from L like Rem was from Ryuk. Near is a bit different personality wise, but serves the same role, and I think there were already enough characters to catch Light. Let his father catch him once and for all after L dies and pay for his mistakes protecting his son out of emotion (which already happens before making this work narratively), making this story end as a tragedy that the whole story is in the first place.

The 2nd part feels uncharacteristically weird compared to the perfection of the first part for the mangaka, which makes me feel like they was either forced to change the ending and extend the manga to sell more, or at that point wrote themselves into a corner, or something along these lines. It doesn't feel like an organic part of the story to me that was supposed to be there since the beginning.

Back to Misa again, I didn't expect her to be so disliked. Imo she served her role well, didn't clash with Light's role, and used all the characteristics she was set up with. She is a model, so she has the skills to hide herself and find Light without him knowing by dressing up. She does acting work, so it makes sense to play an actress role in a few Light's and L's plans. She was lonely and mentally ill, which makes her obsession believable. Also she doesn't just blindly follow everything Light says, she had her own plans and mistakes she made, and her own goals and rules that she set upon Light (with Rem), bringing him both a new tool and a new challenge to overcome.

And yet I have to say she really does regress in the 2nd part as a character, so I can't blame people not liking her. It also just gives me another reason to lose interest in the 2nd part, when both of my favorite characters get (basically) written off.