OK I'm gonna propose that Crawdaunt be added to the threatlist. I think some people, me included, may have nudged at it before but never got a response so I'm gonna try to make a serious pitch.
Looking over your analysis, I think you make a lot of great points. The combination of Adaptability, Dragon Dance, and dual typing gives it the potential to unleash a wall tearing sweep. Dark and Water make solid coverage. Another interesting point you missed in your analysis is while Crawdaunt's defenses are pretty bad, and his typing actually gives some pretty useful x2 resists. The biggest x2 resists, which is a feature of the RU metagame, are against many priority attacks that would normally stop a similar sweeper cold. The Water/Dark typing protects Crawdaunt against Sucker Punch, Aqua Jet, Shadow Sneak, Bullet Punch, and Ice Shard (though those last two hardly come up in any seriousness in this tier.) While Crawdaunt is weak against both Mach Punch and Vacuum Wave, the lack of powerful and popular users in RU gives Crawdaunt additional defenses against a priority stop.
Unfortunately, my difficulties with Crawdaunt that I've experienced is the difficulty in setting up, the difficulty in switching in, the weakness against revenge killing, and the dominance of bug sweepers. Crawdaunt's poor special defense and HP means he is stuck coming in on defensive pokemon, Cressila being the best example that you pointed out in your pitch. While he is immune to psychic, and resists six types, his poor durability makes switching into even resisted attacks bad for his health, and many pokemon, instead of fleeing from a switched in Crawdaunt, will opt to smash it into the ground before it can get a dragon dance off.
Speed Boost Yanmega can run down +1 DD Crawdaunt after a protect, or if it suspects the opponent will try and get another DD off to safeguard against a Speed Boost rundown, bug buzz will finish the crawdad. Both Jolly Durant and Timid Galvantula are just barely above that 342 speed benchmark, and can revenge kill with their STABs of Choice (Though without hone claws, there is a chance that Durant will miss, and with a Life Orb, Crawdaunt can 1HKO Durant with Waterfall. However, it can easily survive Crunch, so unlike Galvantula, it can actually switch into Crawdaunt with proper prediction). Choice Scarf users with a higher base speed than 55 outruns Crawdaunt, and can revenge him effectively. Tangela can wall it, and can stop the sweep cold.
That said, Crawdaunt can work well as a late game sweeper, when the other threats have been dealt with, but I feel that he needs a lot of support from his team, and until the threats have been cleared away, sending out Crawdaunt is a very risky prospect.