The thing with those enermax are that they are multi rail 12v, instead of single rail 12v like your seasonic. If two of your pcie power cables were plugged into the same rail I could see it hitting the limits of that rail with something as beefy as a 6900xt or 3090.
Since you are using an 11900k, with 11th gen there is noticeable performance degradation if you're just doing XMP on your kit since it'll set gear 2 for 4000mhz. This sets your memory controller at 1000mhz only.
Gear 1 100:133 3733mhz will be considerably faster and you should be able to lower timings a bit more at the lower clock. This sets your memory controller at 1866mhz.
Seems like a common issue according to the top review of that keyboard on amazon. It is a hot swap keyboard and they were able to fix it by replacing the switch.
Silicon varies but so does the program you're stress testing the undervolt with. For example in something old like heaven (what optimum tech tested with) I could do 1920mhz on 0.85v and be "stable" all day. But in a game that maxes out the rtx cores like metro exodus I had to dial it all the way down to 1800mhz on 0.825v.
Doesn't matter, you don't need cables that are single connectors. Only difference would be that it'll look more tidy without extra connectors hanging off.
You don't have to do any of that, all you need to do is enable the XMP profile if you're using intel. If you're using amd they should have a similar equivalent.
It's actually even faster than that, it's 14,000mhz effective since it's GDDR. GDDR6X can even clock to 22,000mhz. Your issues aren't due to the memory clock.
The thermal paste looks fine. If you're using the stock cooler that came with it then those temperatures are fine, it's just not that great but it's fine. If you want it cooler or quieter you could upgrade to a new cooler.