Reserved for Pokemon who can sweep or wall significant portions of the metagame, but require some support or have some flaws that prevents them from doing this consistently. Supporting Pokemon in this rank may give opponents free turns or cannot create free turns easily themselves, but can still do their job most of the time.
Reading this requirement for A-rank, Jynx fits it perfectly. Regardless of its flaws (poor bulk, and it's speed is better than "only decent"), Jynx is very good at sweeping large portions of the metagame. An immunity to water-attacks, a good base 95 speed for NU, access to nasty plot, a great base 115 special attack, and lovely kiss to top, make Jynx more than fit for A-. As for lovely kiss, Ferroseed is the best grass-type in this tier, and specially defensive easily manages to shrug off an ice beam. In this case, lovely kiss hitting grass-types is very important. On other grass-types that it can OHKO, such as lilligant, Jynx can proceed to put them to sleep in order to set up a free sub, and KO them the turn after.
As for Dusknoir, I'd be fine with anything in the C-range, I'd just like to defend myself a bit in saying dusknoir takes on kabutops. Dusknoir lives any hit from adamant life orb kabutops at up to 66% (disregarding knock off which it rarely has room for), and always OHKOs in return with earthquake. Shadow sneak does up to 40% to kabutops, and can easily 2hko any life orb weak armor variants, as well as getting off about 90% on any life orb kabutops in general, although you'd have to sack dusknoir in the process. I think Noir is best suited at C, and I've already stated the reasons why. It's niche isn't bad, and, on the right team, it can prove to be an effective pokemon.
Prinplup can wall certain fighting types:
252 Atk Primeape Close Combat vs. 248 HP / 252+ Def Eviolite Prinplup: 103-123 (31.1 - 37.1%) -- 77.8% chance to 3HKO
252+ Atk Hariyama Close Combat vs. 248 HP / 252+ Def Eviolite Prinplup: 124-147 (37.4 - 44.4%) -- guaranteed 3HKO
252+ Atk Gurdurr Drain Punch vs. 248 HP / 252+ Def Eviolite Prinplup: 72-85 (21.7 - 25.6%) -- 1% chance to 4HKO
Obviously this doesn't classify as hard-stopping them, however, it was worth pointing out that the lack of steel typing allowed prinplup to take them on better. As for ground types, I was referring to Seismitoad, Golurk, and Sandslash, all of which Prinplup takes very well. Most of your post refers to Lumineon being superior, but honestly, lumineon is heavily outclassed by prinplup. Lumineon has storm drain and u-turn, but Prinplup has far superior bulk, superior special attack, access to stealth rock, and a similar water-resistance. Even with Lumineon's leftovers, the bulk difference between the two is still heavily in favor of Prinplup without leftovers. Stealth rock is a huge factor, Prinplup being one of only 2 pokemon in NU to get defog and stealth rock. Finally, you said "For the record, Prinplup takes 12% from SR while Leftovers Defoggers takes ~18%," but that makes no sense. Leftovers defoggers take 6.25%, not 18%, and if they did take 18%, that wouldn't be an advantage over prinplup at all. Prinplup, although it has similar stats, severely outclasses Lumineon in most regards, and I don't think it's a fair comparison. I'm still supporting Prinplup in the C-range.