1. In the past as per I have learned from this forum, we have frozen out some XRP addresses so that the holder were unable to move their coins. I am sure still they can lock out our funds if they prefer.

2. Sometimes back they charged up to $20 for one XRP address to hold XRP. But, not sure whether it was only on few exchanges nor how it was implemented.

3. Basically no mining is possible. Mining is one method for how other decentralized systems are making sure about not having one central authority.

4. XRP devs (or some higher officials) keep claiming that their system is more decentralized than any other crypto but never seen any proof. When it is really decentralized then there will be no need to keep on emphasizing the same.