it would be great if next time you could post questions regarding multiple topics one by one. Then its easier for others to find a solution when they have the same question.
1. Is it possible to replace existing OPC UA software with other OPC software
Yes. RevPi is an open Linux system. You can use whatever is available for raspbian.
2. Is it possible to have a layer for Security and also a layer for Redfish APIs
As I don't have experience with redfish and I'm not too deep in DevOps, I can't say. But maybe NicoB knows more. Let me ask him.
3. How can we achieve software and hardware redundency in RevPi
To answer this, we'd need to know, what exactly you are doing. RevPi can be used in a variety of ways. And for each usage-scenario the answer would be different.
4. How we can do remote device management ?
Could you please be a bit more specific what exactly you mean by "remote devide management"?