Scarf Kyurem and Hydreigon are in fact well-known sets. The Megas are assuming that they are already Mega Evolved and just saying that they MAY pose a threat (same goes for all the others I listed). Entei always runs Stone Edge and is a huge threat to an Adamant Charizard without a Dragon Dance up, Porygon-Z is not as irrelevant as you make it and Choice Specs Porygon-Z will often run a Timid nature. Rotom will hurt Charizard X with Hydro Pump (252+ SpA Rotom-W Hydro Pump vs. 0 HP / 4 SpD Mega Charizard X: 195-229 (65.6 - 77.1%) -- guaranteed 2HKO) and only Outrage will OHKO a Rotom-W uninvested in Defense. I will agree with you about offensive Mew however, as I have only seen two people other than myself using it.
http://sim.smogon.com:8080/Stats/2014-03/moveset/ou-1760.0.txt
Kyurem-B uses Choice Scarf over 11% of the time and Kyurem uses Choice Scarf over 26% of the time.
Choice Scarf Hydreigon is its second-most popular set at over 23% usage.
Modest Porygon-Z is more common than Timid, but Timid still takes up a large amount of the sets and Choice Scarf Porygon-Z is its most common set.
Do we have to go over this again?
Let's see.
Entei's only reason to ever be used is to spam Sacred Fire. It being locked (I'm assuming Entei is using it's most viable and only set other than the not-common AV set, which is CB) onto Stone Edge then that's bad for Entei, otherwise, it's extreme set-up bait for CharX. And either way, they'll always tie because Entei is always adamant. Porygon-Z is actually irrelevant so I'll skip it, but I'll give you points for this one, Timid does outspeed. Rotom-W won't outspeed CharX if it's running Modest, and it won't outspeed CharX at +1 if it's a scarf with modest. And no, Outrage at +1 does indeed OHKO all variants of Rotom-W, while only missing a guaranteed OHKO on physically defensive.. which still has a high chance of being OHKO'd:
+1 252+ Atk Tough Claws Mega Charizard X Outrage vs. 252 HP / 252+ Def Rotom-W: 295-348 (97 - 114.4%) -- 81.3% chance to OHKO
(and js: rotom-w normally doesn't have 252/252+, it takes some of both EVs to invest into speed to outrun Jolly Azumarill, so, taking that into account, this is the most absolute relevant calc:
+1 252+ Atk Tough Claws Mega Charizard X Outrage vs. 248 HP / 216+ Def Rotom-W: 304-358 (100.3 - 118.1%) -- guaranteed OHKO
But I posted the one above for worst case scenarios where rotom-w could be running 252/252+)
Kyurem-B's most common and relevant set rn is mixed, and as you said Scarf is only 11%, so I don't see how that's an issue. It's easy to figure out what set Kyurem is running purely by seeing how much damage the first attack it did made. If it was Outrage/D.Claw and it did a lot then it's band. If it's Substitute, then it's mixed. If you see 10% HP reduction after using a move, it's obviously LO. If it's too weak however, then it's Scarf. And regular Kyurem isn't relevant, but yeah I guess Scarf is its most common set in OU, props to that.