An owner witnesses with his/her respective *.skey. The *.vkey is a public key that can be used for verification. Yes, you send the tx.raw around to get witnessed. They can do this with ADALite.
Notice, how the above uses cardano-hw-cli instead of cardano-cli. Generally, if we are talking about more ADA than you are happy to loose, you want to use a HW wallet. The pool’s cold key can nowerdays also be secured with a HW wallet.
All pool owners contribute to the pledge. It is however a property of the pool config how much the pool actually pledges - this is not automatic. As a consequence, a pool owner cannot just leave and delegate elsewhere. If the pool’s pledge is not met anymore, there will be no rewards for nobody, also not for the delegators.
If an owner wants to leave, the process is as follows …
- update the pool config to reduce the pledge
- wait until epoch N+3 (not just N+2)
- update the pool config again to remove the owner
Note, this also requires sufficient trust.