Forgive me because I don't understand what's going on still.
Yes. Stealth Rock is absolutely essential in any match, meaning that you will want to have them up for most of the match, which is simply impossible if your Stealth Rock setter is Choice Scarf Landorus-T. Meanwhile, Defog is only used for emergencies, most likely alongside another Defogger, which should make up for Landorus-T's lack of reliability when Defogging.
you forgot leftovers in your lele calc. tapu lele has only a 14.1% chance to ohko and you can't go more spdef than that because of the mega diancie calc you shared yourself. honestly, the main concern here is that mew only has so much bulk, and you really want to outspeed heatran. i suppose you could run 72 speed instead for modest heatran or something, but that point you're completely banking on the fact that your opponent is not timid, which may not be ideal. honestly though, a 14.1% chance ain't too bad because hazards shouldn't be up if you're playing well enough and you can always pivot into another pokémon once you know what tapu lele is locked into, you can go mega aggron if you see a choice specs moonblast for example, not to mention that, leftovers + soft-boiled always heals more than specs moonblast, so if they get a roll of 183 or lower, you can just keep soft-boiling and lele won't be able to ko you unless it crits, that way you can get healthy enough to attack. overall it's a niche fit on stall and it fills a role very few pokémon can.
252 Atk Garchomp Outrage vs. 0 HP / 4 Def Tornadus: 262-309 (87.6 - 103.3%) -- 18.8% chance to OHKO
I feel the majority of your questions can be answered by careful dissection of your replays and, as Leo has shown, using a damage calculator.
Yes, very much so infact. The ability to check grass-types that threaten rain teams, most notably Kartana, combined with the offensive potency of 100% accurate Hurricane makes Tornadus-T a boon for rain teams
As unfortunate as it is to say, OUPL is a tour that isn't taken very seriously so usage stats and mons used in that tour shouldn't really be considered. For example, Castform was used in week 1 of that tour and ended up winning, but that has nothing to do with how good it is. Official tournaments like Smogon Tour and World Cup of Pokemon are much better indicators of the current metagame than OUPL.
that's a gamble. 33% chance they hit themselves. not worth it at all when thunder wave descreases speed.
I think this question is better suited for the Monotype section.
