Why did Feathers the penguin choose Wallace? Because he knew that not only is Wallace a ditz, but he's an incredibly deep sleeper.
Additionally, Feathers very quickly sets upon his plan once he enters Wallace and Gromit's house. How did he know he had all the means for his scheme there? Because he likely saw the Techno Trousers take Gromit for a walk earlier. Additionally he likely also saw Gromit sneaking from out its grasp at the park, clueing in that he would be too clever to his dupe, and therefore had to be gotten out of the way.
In the first comic book, "The Lost Slipper", Wallace builds a time machine in order to go back to earlier that morning, but accidentally ends up in the Battle of Hastings. Well, the exact time he was trying to go back to was 11:06, the setting probably started at 1000 and he added 66 of what he thought were minutes, ending with 1066.
Wallace mentions the techno-trousers are "ex-NASA", but what would NASA want with robotic trousers? Arms for heavy-lifting, a helmet for breathing in space, but trousers? Then it hit me... ex-NASA. No longer belonging to NASA. NASA couldn't find a use for them, so they just got rid of them. And what use does Wallace have for them? Literally nothing other than having them walk his dog for him. They are so utterly useless that no-one other than a determined criminal can think of a purpose for them, and once the whole ordeal is over, Wallace and Gromit just toss them in the bin and forget all about them.
Nearly all the villains demonstrate a high level of cruel spite towards Gromit, even beyond what their plan necessitates. This makes sense given most of them turn out to be savvy to him being Wallace's Hyper-Competent Sidekick all along. They act petty towards Gromit for making what should be a simple scheme far more complicated than they'd prefer.
Based on some of the Were-Rabbit behavior, specifically at the end when he got angry at Victor Quartermaine hurting Lady Tottington and how he recognizes Gromit, there is a chance that it's a Superpowered Evil Side for Wallace at best where he loses himself to his rabbit's instinct or at worst an And I Must Scream situation where he could see and know everything that happens around him yet can't do anything until he fought back against it.
Imagine what will happen if Wallace went to Piella's home instead of Gromit?
Let's just say Gromit would never see Wallace again.
Piella's abuse of Fluffles wasn't just because Bad People Abuse Animals. It's implied that Fluffles actually tried to stop Piella's crimes earlier and was even against her plans from the start. When Fluffles intervenes once Piella tries to murder Wallace, Piella yells "You've crossed me once too often".
After Piella's first attempt to murder Wallace is thwarted thanks to her being hit with a sack of flour, Piella angrily takes Fluffles home saying that she wants a word with her. It's very possible that Piella thought Fluffles was responsible for the flour.
