Is OpenOpcUa stack compliant with other UA Stack ?
Yes, OpenOpcUa stack can be used with .Net, Java or ANSI Stack from OPC Foundation and toolkit vendor.
Are there abstraction layers for threading and sockets?
Both and also for string, mutex, semaphore
Are you using the OPC foundation’s stack as basis?
Yes. I fork it 12 years ago. I fixed a lot of bugs and enhance it on many aspect. The OPC Foundation provided originally a Windows only stack. Their Linux layer is OpenOpcUa stack.