The second you compared it to farfetch'd your argument was completely moot. Farfetch'd is a horrible pokemon regardless of what kind of support you bring to the table. Even so, no matter what support you gave it it would be a worse generic flying-normal type than every other one in the tier anyway. Also, the reason people want shedinja to be a D+ in the first place is that it needs so much support. Shedinja also has a niche that literally no other pokemon in the whole game-- not just the RU tier-- can fulfill. If shedinja didn't need to worry about spikes or stealth rock obviously it would be much higher such as a B or an A, but the whole reason they're attempting to make it a D+ is due to the fact that its massive necessary support is a hindrance.
I know that Farfetch'd is a terrible Pokemon, however it can still work with a hell of a lot of support. Replace Farfetch'd with Shedinja and you basically have a description of Shedinja. The whole point of the comparison is that Farfetch'd actually needs less support from Shedinja in a way, one-Pokemon support in Ninjask, and it can sweep against the right team match up. Shedinja can't boast this, as it also needs a Rapid Spinner, type resists, lures, rest of the team etc. etc.. Also, "it would be a worse generic flying-normal type than every other one in the tier anyway" is something that I understand, however look at all Bug- and Ghost-Type Pokemon ranked on the tier list:
The lowest Bug-Type Pokemon is Ninjask, and the lowest Ghost-Type Pokemon is Dusknoir. Both Ninjask and Dusknoir are better than Shedinja; this doesn't mean that Ninjask and Dusknoir are good, it means that Shedinja is worse.
Ok for your second points I'm not quite sure you know how this system works. We are not ranking every random Pokemon, we're not going to have Ponyta and Happiny in Low D at some point, we are ranking Pokemon that are good, possibly better than just viable. D is for the odd Pokemon that have odd niches, such as Natu or Ditto, Low D is for the Pokemon who generally fail to give the support but the team isn't built around them, the majority of D-Rank Pokemon are support Pokemon. Ninjask, Natu, Audino and Dusknoir for example, these don't have teams built around them, they are part of the support a team uses. In fact, no Pokemon on the rankings has a whole team built around them besides Riolu, who actually has a pretty low-risk high-reward strategy if you can use it correctly. Shedinja can boast none of these things, and thus should not be ranked. Also, saying that without a hazards weakness Shedinja would be A or B rank really shows that you have no idea what's going on...