What you have here is **a disagreement**. You prefer doing things one way, the technical team prefers their way. So the way to fix this is to ask **WHY?**. And not just why they prefer their way, but also why you prefer your way.

Maybe they are set in their ways, and you are set in yours. Maybe they don't understand all this Agile thing and don't see the point. Maybe Scrum seems dumb. Maybe they don't like the way you split the stories. Maybe you are actually bad at spliting the stories. Maybe they have some insight into the product and think it's better to do things their way. You are the PO but maybe you should be more open to their feedback. Maybe they are not very skilled technically and they are worried that they will make a mess of things by not knowing how to split work properly, to allow for incremental development, so they try to keep everything together. A lot of "maybe"s because I'm trying to guess what's happening simply from what has been posted here, but I'm sure you might have made similar **assumptions before reaching a conclusion** and asking this question.

So organize a meeting with everyone and discuss things. The purpose of this meeting is to understand each other and [**get to the bottom of the problem**][1]. Only then can you find **a solution that works for everyone**. Telling them you want them to work in a more Agile way won't mean anything to them unless they understand why it's needed.

The SM/Agile coach can mediate things and make sure talks stay at a proper, respectable level, but this needs to be a separate meeting, not part of the Scrum events. **The retrospective is the place to have such discussions, but it's obvious from the OPs question that retrospectives aren't doing their job properly** (the team goes back to their old ways the minute the SM isn't looking, there is opposition to the idea, and this has been going on for a long time, so much so that the OP has given up and is willing to work with mini-waterfalls despite the risk to the project and the customer). A separate meeting is extra signalling about the weight of the situation. People need to understand that "**this arrangement isn't working out for everyone**". Once people understand the weight of the situation, the problem is decomposed, and the root causes of the disagreement are found, only then can something be done about it. Otherwise anyone, on both sides, can perceive things like "**it's because someone says so or wants so**".


  [1]: https://en.m.wikipedia.org/wiki/Five_whys