Sounds like your M6 bolt for the horn repositioning might have been too long.
As for the grille, I wonder if you were trying the wrong way to remove these clips ? You just need to turn then slightly, then they pop up and you pull them straight out. To reattach them you just push them in, shouldn't break at all.
The only thing that I found tricky on the sports grille is getting the bottom edge of it to engage properly, since you need to pull up on the existing plastic bits while you slide the grille in at the bottom, and there's not much room to get your hand down there. You can install it without doing this but it won't be attached properly.
I can give you a hand if you like, since you're not too far from me. I imagine you can get some replacement clips from Toyota/Lexus.