There is no cap, it is just bound by the amount of WETH in the contract. At the moment, Origin is the only LP for the ARM, but soon we will be opening it up for external LPs to provide their ETH. This should happen in October of this year.
If the total amount needed is not available in the ARM, async redemptions to ETH are 1:1, there should be no fee. The Instant vault redemption that you are referencing in the docs is the old exit method that is hardly used nowadays.
