Can there be multple soffice processes per user per host ?
Answer : There can only be one soffice process per user per host, if one soffice process is running and you try to open a document with another soffice command via terminal, this request will be transfered to the already running soffice process and the document will be opened in a new window of the same process. However while an instance of soffice gui instance any number of headless soffice commands can be run. This is probably because the headless processes need only readonly access to source files and cfg.
Can there be extensions installed just for a particular document or can there be any correspondence between extensions and the documents open by soffice ?
Answer : An installed extension is common to all documents opened by soffice, there is no concept of per document extension. The extension should be prepared to handle the presence of multiple documents/ or calls from them.
Is there any easy method to see the states of calc process while running without using gdb ?
Answer : As far as we know there is no such way other than using gdb.