如何在WPF / C＃中创建自定义MultiSelector / ItemsControl

I'm trying to create a diagramming application in C# / WPF. What I going for is somewhat similar to Microsoft Visio although I'm not trying to clone it. I kind of wrote this question as I was coding and just put all the issues I had into it in case someone will find it useful. Maybe I've been thinking too hard but I feel like I could throw up on my keyboard and produce better code, so feel free to give any suggestions on every detail you catch (grammar excluded :-))I'm trying to create a diagramming application