OP, part of the problem you are having may be that you aren’t communicating well exactly what happened to the rental car place. The experts here are having a difficult time understanding exactly what happened likewise. I’m not criticizing, just pointing out this may be causing the rental company to misunderstand what happened, and think you caused the damage to the clutch.
May I suggest that you secure an intermediary to communicate with the rental car company going forward, someone who is able to unambiguously state your own position. Once everyone agrees on the facts, they may agree it was just an unusual failure mode and not put the blame on you.
BOGUS! You know how to drive a manual transmission, sounds like a failure not due to your use. Now how to deal with it, my first stab at the cat would be asking what was the exact diagnosis of the failure. Assuming rental insurance was not taken out, many credit cards include it automatically, see if yours did, is this a major company? Call to next level in order.
Coasting down a grade in neutral and playing this in and out game with the clutch will destroy the clutch disc. I think the shop has the advantage of being able to smell the failure. With the clutch in, the engine returns to idle speed and the clutch disc spins at perhaps 4000 RPMs depending on the gear selected. To gently release the clutch during these conditions would be like a 4000 RPM take off from a stop. To repeat this several times would destroy the clutch. Shift from sixth to fifth or fifth to forth when necessary, do not use the clutch for more the a fraction of a second.
Nevada_545: Eureka! I think you may have untangled this “web”! I am much less confused. I have never witnessed or heard of the technique of disengaging re-engaging the clutch to control engine braking on a descent. OP does disclose driving “big trucks” where downhill brake management is very important. A 2015 VW Golf with 2,900lbs curb weight is very different. Choose an appropriate gear for the descent and adjust speed with the throttle or brakes. You are 100% correct that adjusting speed with the frequently engaged/disengaged clutch is the same as multiple “jack rabbit” starts which is a sure way of destroying a clutch and causing wear/damage to the entire powertrain plus tires. The $2,000 seems a bit high for a standard disc, pressure plate, release bearing but if the flywheel was a very pretty burned blue it seems reasonable.
If they’re going to blame the OP for causing this damage then they should be well prepared with a technical explanation of how this happened and produce the failed parts which could be examined by a 3rd party without a vested interest in the outcome.
I can understand a clutch becoming inoperative due to a slave cylinder failure but certainly don’t see how a slave cylinder can cause the clutch to disentegrate and damage the transmission.
If the slave cylinder was the alleged cause then that should not take more than a minute to verify the slave’s operation; assuming one buys into the slave cylinder cause and which I don’t.
So with all of the manual transmission cars and trucks that take an absolute flogging in multiple mountain ranges it would seem to me that cars and trucks should be dying in droves; especially in the hands of many inexperience manual trans drivers.
The only theory I can come up with is if the slave piston stuck in the disengaged position and constant pressure was applied to the throwout bearing until the TO bearing disentegrated from heat. This means the pistons should be stuck in the bore and the TO bearing (or remains) should have a blue color from excessive heat.
If this is the case it’s not the OP’s fault.
And this is the crux of the issue, because the mechanics did not save the parts that were replaced even though they knew there was a question as to who was responsible for the part failures.
This strongly suggests to me that the rental car company knows it isn’t OP’s fault and doesn’t want to keep evidence that proves it.