1) You will encounter enough Heat Wave targets that are neutral to it and Metal Sound helps you turn 2HKOs into OHKOs to make the most out of your limited Trick Room turns. Metal Sound is pretty much Bronzong's way of having an offensive presence, like many other Pokémon try to do with Helping Hand.
2) Yeah, that's why Choice Scarf Landorus-T, the most common moveset on the second most common Pokémon rarely uses Earthquake*. An Earthquake that isn't even boosted by it's Choice item or it's ability. Adding Flying/Levitate/Telepathy Pokémon to your team works for Landorus-T, but the same thing can be said about Sludge Wave and Steel/Telepathy Pokémon. Having multiple Steel-types in a bring 6 pick 4 format, where Mega Kangaskhan is the most common Pokémon and Hyper Voice Sylveon is the third most common Pokemon, isn't that bad of an idea. Adding another Ground-weakness might be, but much like Bronzong, not all Steel-types are weak to Ground-type attacks...
3) If Sludge Wave poisons your partner and that is a big IF, since you are the one who decides when it's used, you are right and might end up being 2HKOed, but it also turns 2HKOs/3HKOs into OHKOs/2HKOs FOR you, which will help you a lot more often than it hurts you, because like I said you decide when to use it and it also hits both of the opponents Pokémon, while only one of yours... in the worst case.
*obvious irony is obvious