Yes, well aware of all that. Notice I never said anything about charging at 350 kW, only paying for the 350 kW price tier which Rivian would fall in even if power is tapered off from the originally advertised 160 kW peak.Keep in mind that you will not be able to charge your Rivian at 350 kW even though you are at a 350 kW capable charger. Those can charge 800 V vehicles at 350 kW but not 400V ones until their manufacturers implement a series/parallel capability like the one Rivian has patented or install DC/DC converters (unlikely IMO). I believe the most CCS allows for is 500 A which, for a 400 V battery means a maximum of 200 kW. In addition to that the car will taper the charge rate as the battery SoC gets higher so it takes longer to add 1 kWh at the end of charge than at the beginning. Be sure to take this into account when comparing the two schemes.
Per kWh charging is much fairer in the sense that one is not charged for power he can't take nor is he penalized for the peculiarities of his cars taper algorithm. What is not so fair is that people in states where electricity is much cheaper than average pay the same as states where it is more expensive.
I am however assuming that EA's price tier is based on peak power and that they would not drop you into a lower price tier if you dropped below 90 kW due to taper during the charging session. That may not be a correct assumption. We also don't know just exactly how Rivan will manage power during a charge to know if that detail is even relevant to this discussion.
The basis of my cost comparison relies on charge times supplied by Rivian (as reported by MT here) so that I don't have to make my own assumptions about actual peaks and taper curves. Rivian stated early on that a 5-80% charge could be completed in about 50 minutes for the 180 kWh pack. Also assuming no EA subscription.
50 minutes @ $0.32 = $16
180 kWh * 75% * $0.43 = $58.05
Looks to me like per minute is far cheaper than per kWh in this scenario.