Choose your battles carefully
You mention you are new to the team and new to the organization. It will take some time before the team fully trusts you and your judgement. If that has already happened then you have more arguing power to support your point of view. If you are still very new, then have patience and wait a while. It's also very important to measure your words when arguing your case. If you used words like "stop this madness", like you used in this question, your message content may have been dismissed and attention only payed to the way you transmitted it. Combine that with the fact that you are new and your opinion may be ignored until you truly become one of the team.
My suggestion would be to let it slide this once. You said what you had to say, you didn't manage to convince them for now, just accept and continue with your work in the new sprint.
Use the next retrospectives
Having the sprint planning and the retrospective reversed can cause waste and it can be inefficient. But there is a difference between "it can" and "it is". Have the meetings as instructed then at the next retrospective raise these two points:
- the retrospective should be for the sprint like a dot is for the end of a sentence. Having planning makes you think and sets a context for a new sprint. Then you go to work on the new sprint and then, the next day, you need to do a context-switching and discuss the old sprint. This is inefficient. To keep with the analogy, you are setting the dot in the middle of a sentence like this.
- let the "mini-planning" extra meeting occur then at the retrospective mention that it caused waste and propose again to have the meetings in their proper order. If the waste doesn't occur, just wait until it does, then raise the issue.
You should then all discuss ways to improve these points at the retrospective, with the obvious choice of switching the meetings. If everyone in the team really wants to improve things, they will do the meetings like described in the Scrum Guide. And like I said, chose your words and your battles carefully. Make it about improvement of the work. You don't want to make it about you being right and them being wrong because if that happens you'll have an event harder time of convincing anyone of anything.