1. The SurfacePro is the only convertible capable of doing high end work such as 3D modelling. Everything else should be up to surfing and word processing.
2. I just did some looking around and came across the Lenovo Tablet 10. It looks pretty good, better than the Miix.
https://www3.lenovo.com/gb/en/lenovo.../p/22TD2LV1010
3. You need to think of these machines as sealed boxes. They are made small using lots of glue and are essentially unserviceable.
4. This isn't possible. The only option I can think of is to have a Windows machine always on at home then access it via RemoteDesktop/TeamViewer on the iPad. The only remote desktop sharing app which supports a mouse is by Citrix and it requires a pro license.
IMO you already have the best device for your requirements in the SurfacePro.