A few comments..
1. If you are interested in the full technical details, see this
http://upnp.org/specs/arch/UPnP-arch-DeviceArchitecture-v2.0.pdf2. The 255.255.255.255 and 224.0.0.241 addresses are nothing to do with UPnP so I can't speak for them at all. (Except to say that if a UPnP device is using those addresses, then it is broken..)
3. A note on terminology. The UPnP standards define the basic architecture of discovery command and control for ALL classes of UPnP devices, plus also the command and control syntax for SPECIFIC UPnP device classes (such as Digital Media Server or Digital Media Player). The DLNA is a guideline that builds on top of UPnP to provide more specific constraints to improve interoperability or DMS and DMR devices.A UPnP device need not be DLNA compliant, but a DLNA device MUST be UPnP compliant..
4. Unfortunately I don't know if Chromecast has any UPnP (or DLNA) functionality. But possibly it does not..
5. Your last question seems to imply that having a mixed topology network (e.g. a mix of wifi, Ethernet, PLC or whatever), requires you to use several sub-nets. That is actually not true, since it is perfectly possible to have a mixed topology on a single sub-net. (This is what I have).