1. MikeDawg's Mamoswine: HP Fire does indeed give you a good lure, but having the same typing (ground / ground) isn't really something I want to do. With Keldeo, Starmie, Rotom-W, and Rain Teams running a good portion of the metagame, I feel we'd be hard pressed against them. Granted, you could fix this in later building, but that could be said about any of our submissions this early in the submission process. I feel like we should use a partner that can defeat most of Lando's counters, not a certain portion.
I don't see the need to lure out Ferrothorn when setting up on it with Landorus-T is super easy once you pull off an Intimidate. Granted, luring out Skarmory is pretty nice, but I'm sure other Pokemon could do this better. Keep in mind the Mamoswine adds a Rotom-W weakness, Starmie weakness, Keldeo weakness, and rain weakness that Landorus-T already has issues facing.
I really don't think typing is as big of a deal as people believe it to be. If we are facing a keldeo, weak, neutral, or resistant, it is quite likely that we will get kod.
And, as mentioned, since lando as a pokemon is quite offensive, I believe that offensive synergy at this point is far more important than defensive; hit before we get hit.
And I don't think lando is as helpless against those rain threats as you think. Especially if mamo is able to weaken the aforementioned pokes, lando is much more comfortable just setting up a rock polish (or both!). I really think we should be using a life orb set though. Leftovers puts too much pressure on lando to get a swords dance because of how "weak" it is
252+ Atk Life Orb Landorus-T Earthquake vs. 4 HP / 0 Def Keldeo: 277-328 (85.49 - 101.23%) -- 50% chance to OHKO after Stealth Rock
252+ Atk Life Orb Landorus-T Stone Edge vs. 248 HP / 0 Def Rotom-W: 160-188 (52.8 - 62.04%) -- guaranteed 2HKO after Stealth Rock
252+ Atk Life Orb Landorus-T Earthquake vs. 252 HP / 0 Def Politoed: 320-378 (83.33 - 98.43%) -- 68.75% chance to OHKO after Stealth Rock
as opposed to
252+ Atk Landorus-T Earthquake vs. 0 HP / 0 Def Starmie: 223-264 (85.44 - 101.14%) -- 87.5% chance to OHKO after Stealth Rock
252+ Atk Landorus-T Earthquake vs. 4 HP / 0 Def Keldeo: 213-252 (65.74 - 77.77%) -- guaranteed 2HKO after Stealth Rock
252+ Atk Landorus-T Stone Edge vs. 248 HP / 0 Def Rotom-W: 123-145 (40.59 - 47.85%) -- 8.2% chance to 2HKO after Stealth Rock
252+ Atk Landorus-T Earthquake vs. 252 HP / 0 Def Politoed: 246-291 (64.06 - 75.78%) -- guaranteed 2HKO after Stealth Rock
As you can see, though, we are not exactly helpless to those pokes that you mentioned anyway. But as you can also see, switching to life orb brings such a huge amount of kos without swords dance that it seems silly not to :/
And you yourself mentioned the biggest draw to mamo: it's ability to lure skarmory and the like.
What other pokemon can do that as mamo does, exactly? None of those pokes are remotely afraid of switching in to mamoswine, meaning they are quite likely to do so.