19. If I deposit underlying asset into an option underwriting pool, could I withdraw it anytime?

Yes. You could withdraw your deposit any time you want, as long as it doesn’t exceed the Net Withdrawable Amount(NWA). NWA is the amount of asset in the underwriting pool that is available for withdraw, which changes from time to time along with the net inflow/outflow of asset into/out of the underwriting pool.
Another way for underwriters to exchange for liquidity is by selling their underwriter tokens to someone else on the market.