Good Morning, Zach:
It's been a very long time since I had a Well Tempered (one of the first iterations and very good at the time). From your pics and videos, the hum and mosquito(?) buzzing do not appear/sound to be a by-product of either phono stage.
1. On the aluminum/metal RCA block, the ground post would be (should be) in contact with the aluminum/metal block and the ground tabs of the RCAs would also (should be) in contact with the aluminum/metal block (this is assuming that ground post and RCA ground tabs are indeed contacting the metal, i.e., the metal sanded or filed at those contact points).
2. Remove the clamped ground wire you added just so you are back to the original configuration/connections.
3. You can use a multimeter (VOM) and set to to resistance or diode/continuity check to determine if indeed the ground post and the RCA ground tabs give a zero resistance or beep on the continuity check.
4. You can also check that the RCA jacks are isolated from the ground (ground post and RCA ground tabs).
5. Gently disconnect the cartridges clips from the cartridge (if there is a phono cartridge mount and connected) and then test the continuity of from each clip to it's respective wire going to the RCA block. Please remember/mark/note which clip connects to which cartridge pin if not the clips are not color coded with shrink wrap.
6. From the looks of the tonearm wiring, it's not the original and may have been swapped/re=wired at one point in time. (and if so, may be the cause). I believe Bill Firebaugh used sand to filled/damp the tonearm tube of resonances - so, if the wire were indeed changed, then maybe the wires weren't insulated properly or twisted (helically) to reduce crosstalk/hum/buzzing, or maybe a small part of one of the wires is touching the tonearm metal.
7. Try a different set of RCA cables/interconnects (just to rule out the cables and the RCA plugs).
8. Some older cartridges (MM?) may have a slightly different ground technique/shield that may need to be either isolated or in contact with the metal head shell or grounded.
9. Kind of suspecting something to do with the wiring inside the tonearm or else the RCA block assembly.
Hope this helps in narrowing down the culprit/cause.

Thanks.
Sincerely,
Kingsley.