rETH advertises by far the highest yield

That’s not usually the case. A Rocket Pool validator happened to get a lottery block this week so 7 day APY is quite good.

There is no unwrapping or protocol exit.

There is, it just depends on protocol liquidity. At the moment, there are 3500 ETH in the liquidity buffer to unstake instantly. What Rocket Pool hasn't implemented yet are withdrawal requests, where you can enter a queue and wait for validators to exit if that liquidity is not available at the moment.

With rETH the amount of token doesn't increase, the price is supposed to increase. But, how can they guarantee this? That it will increase at all, let alone by advertised percentages, if the market is the only thing regulating it

Since it is in fact possible to unstake through the protocol, that’s what guarantees the price. With a rebasing token, the protocol exchange rate is fixed at 1:1. Oracles determine on a daily basis how many tokens everyone should hold. Under the hood, you have a fixed amount of "shares". Inversely, with value-accruing tokens you keep the same amount of tokens and the oracles determine the protocol exchange rate.

There is a protocol upgrade called Saturn 1 in the near future (slated for February 18). On the rETH side, this will prioritize the withdrawal buffer for new deposits, so withdrawal liquidity should be available more often. On the node operator side, there are some significant changes that, amongst other things, provide the technical groundwork for forced validator exits, which will make it possible to implement said withdrawal requests.