It depends, not every phone does presence right and they all have different levels of support.
The 2 you are using are very different from how the majority of them work (snom, polycom, linksys etc)
Did you try reversing the role of A and B

I will pickup a snom320 and a gxp 20xx and will try

Topic 3: presence seems stateless if is not a phone

to create a module myself that handle
presence using a database?

YES

I looked at source code of conferece/fico (that generate presence from XML config) and dingaling (who use database).
So i think its not difficult to write it.
Hope to have free time :)

Thank you, Antonio (AGX)