Since Development Manager is refusing to acknowledge that integration might be the problem and yet, you're working in Scrum, perhaps it's time to introduce him/her to Nexus?
Nexus from Scrum.org
##Nexus from Scrum.org It'sIt's framework built on top of Scrum to scale it and it's by far scaling framework that focuses mostly on dependencies and integration. Framework can help you holding your ground and defending that this is unavoidable and necessary investment. It might also uncover some additional underlying issues with dependencies (late integration, poor requirements and architecture, etc.).
Immediate next steps in your case
##Immediate next steps in your case GetGet familiar with Nexus, discuss it with your Dev Manager. Having industry experts behind you and your opinion that integration is painful should help you. I wouldn't be surprised if discussion would be enough to acknowledge problem.
When/If you'll establish Nexus with it's Nexus Integration Team you'll have true bandwidth and focus on working through your environment stability and resolving dependencies. It will also become evident that you won't get environment stability and integration issues resolved for free.