Is a software developer entitled to participate to paper?
Based on what you say, I believe ethically you would be listed as an author on the grounds that data analysis in an integral part of research. At least, this is what I would do. I would do this trying to keep things professional even if personally I came to dislike the software developer.

However, in practice, it may depend on the contract you signed. If there was nothing in the contract, things get trickier as the situations seems already deteriorated. Is there anyone in the research group, apart from the IT engineer, that can vouch for your work and argue for your inclusion?

If you did not sign off the copyright to the software, you may have legal grounds for not having the research group use it (again, depends on the contract), although this would be harsh. An alternative would be to publish your own paper on the software, just as a data analysis tool if it is general purpose enough to be of use to others. That would give you even larger credit. Note however that most likely the software would need to be available free of charge to other researchers.

(As a minor point, I do not understand your first bullet point. If you are not on the paper, why is it important to you that the others interpret the results correctly?)