How are you running the 32-bit Explorer?
If I disable Explorer Replacement in Opus and then run 32-bit Explorer using this then it seems to work fine and double-clicks on folders open them within the same 32-bit Explorer window:
%windir%\SysWOW64\explorer.exe /separate, /root, c:Without the /separate I just get another 64-bit Explorer window since the second instance just tells the already-running 64-bit instance to open the folder, as I'd expect.
Regarding Opus startup time, there are some ideas here and here. More generally, is there any reason you didn't let Opus launch once and then stay in the background ready for whenever you need it? It's very fast to open Opus then and Windows will swap it out to disk if the memory is needed for something else.