is it possible to attack patent no. 5,265,261 due to terms of prior art?
I don't think we are concerned about that. At least I am not. It's wasted time as it leads to nothing.
The problem of patents on abstract concepts (so-called "sw patents") isn't addressed by fighting each patent in turn. And the problem of misuse of license contracts and libel ("IPR impairing"), or FUD more in general isn't addressed by addressing this specific patent.
Moreover, if we deal with the details we appear to accept the overall ideas and thus publicly agree to have them explored further.
I express my own points, not those of any organization I belong to.