Is it possible to publish without adviser?
The response to this depends to a degree on the type of institution you were studying at and the norms for that educational system.
In most countries now the norm would be for the Graduate School to have the final say on matters like this. Particularly if the advisor(s) involved are retired (or remaining as semi-retired adjuncts).
Given your OS research was on exchange, the Graduate School at the host institution would typically defer to that in the home country on matters like this.
If the agreement you signed appears to be binding they may discourage you from publishing. If not, I expect they would encourage you to publish. They may even recommend this irrespective of the opinion or the status of the advisor, but that would depend on the culture (and politics) of your home institution.
Graduate Schools often have expertise on these kinds of issues as IP issues are common in collaborative projects. Broadly speaking, most Graduate Schools want their PhD candidates and graduates to be publishing. It's a good look for them and its what they are there to support you to do.
My advice would also to be raise the apparent sharing of your work as a separate issue. Not that I'm saying don't raise it - but if your goal is to be able to publish the results of your research then aim to sort that out as a separate issue.
I would then consider if you think that the sharing of findings with other students looks like the kind of academic misconduct that is defined by your institution and if so consider raising this as a separate matter. I would handle the prospect of academic misconduct with care if you are thinking of raising this.