>> At L2, no difference for battery health.
As noted in post above, L2 is nothing in battery wear compared to DC fast charge.
I doubt it is an issue with Rivian, but for the Hyundai IONIQ 5 which is also rated to 48A L2, there have been a lot of failures of the ICCU module which includes the AC L2 charger, plus some IONIQ 5s had related issues such as hot charging ports on the IONIQ 5. While I always charged my Mach E at 48A without thinking twice, I backed off to 32A (chargepoint flex lets you set the charge current by the analog signal communication with the EV) on the I5 trying to go easy on the charger.
So, some of what you hear in cooling might rather be the AC L2 to HV charge module in the Rivian being cooled. But, if it's engineered well, there would be no concern for longevity of the charger module.
>>the EV charger and the charging cable is warm to the touch
Assuming no bad connections or L2 EVSE defects, this is normal. EVSE always have the cable to the EV sized for use in air. My first Clipper Creek L2 EVSE at 16A to my most recent chargepoints (32A and 48A) are like that. Some even say that you must uncoil the charge cord for use to get proper cooling.
>>My opposition is saying charging at a lower amperage is 75-80% efficient where charging at 48a is 94% efficient
Seems like an alleged "fact" that maybe Rivian could confirm. Otherwise, something you can try to measure. This is a very common question / topic (L2 at 32A, 40A, or 48A, Is one better?) at almost all BEV forums. So you could learn a lot from other forums such as the Mach E forum (a lot of electrical engineers and other high tech types hang out there) or one of the Rivian forums (google shows several threads on L2 charging).
Depending on your skills and available equipment (or, stuff you might buy for the job), you can try to answer some of these questions yourself. There are many ways to measure the energy from your home electrical system to the charge station (the EVSE, the L2 charger is in the BEV). Many charge stations report power consumed, but are often kind of course both in time and resolution. There are any number of energy meters you can connect near your charge station that give very accurate data. For example, a SENSE energy monitor (while pricey) can be mounted with the current pickup coils only on the two 240V "hot" wires to the charger. With only the EVSE as the load, the SENSE power graph is relatively high resolution in both time and power level.
On the Rivian side, you can measure the energy change in kW-hrs gained. However, efficiency is not a trivial measurement, at least because while most of the energy is charging the Rivian HV battery, some is running the low voltage systems including those pumps, computers, telematics, BT, Wifi and the like, etc. Also, some energy loss (probably very little) is warming the cable from the EVSE to Rivian. Again, also as mentioned above, depending on how comfortable you are with measurement equipment, you could measure the L2 voltage into the EVSE, the L2 voltage at the EVSE out into the charge cable, and then the L2 voltage at the Rivian inlet to see the various losses due to resistance (ohmmic loss). It is probably not worth the effort unless you are into it as a hobby electronics academic project.
I was horrified in 2014 when I realized my first PHEV Chevy Volt, Clipper Creek 16A 240V EVSE charge cable was getting warm. But, after working numbers in a spread sheet, I realized that the loss was trivial, both in energy and cost, in fact over years, less than would have been the cost of increasing the size cable to reduce the loss.