10base2 (RG-58) cable can only handle 300 Vdc and will melt at around
10 A, so the available power would be well below 3000 W :-)
Any DMT/OFDM system can carry much more than that. For instance DVB-C2 used by some CATV companies and MATV systems can carry at least 10 Gbit/s on a single coaxial cable with up to 12 bits/symbol, at least
850 MHz, potentially 2 GHz bandwidth. Of course DVB-C2 is highly complex.
RG-174 might be carrying several Gbit/s at those distances using some sort of DMT/OFDM.
The obvious choice would be to just stop worrying about it, because it's not actually an issue. Both problems go away once you realize that there's not actually any hurt in just sending the same data again if the originating node has nothing newer available. Yes, that will mean you waste some of the bandwidth on needless repeats, but then again, SPI is generally pretty fast to begin with, so a little waste should be tolerable.
Hmmm, so we'll have to use separate cabling to heat larger rooms during the cold winter days (no such issue down under I suppose :D ).
My first thought re your suggested cable modem techniques was "too complex". Well complex of course, but perhaps not "too complex" nowadays, I would bet cable modems are single chip units nowadays (actually I opened one some 5-10 years ago and it had some BGA plus some small stuff, IIRC the radio part was at least in part discrete). But my thoughts on that revolve around misusing the RG-174 in the good old 10 Mbps manner, say 50 Mbps should be easily doable nowadays. Hopefully the PHY will not need these -9 volts... but keeping it galvanically isolated makes that irrelevant really. I would also vote for a version with no galvanic isolation, say for use within the same room, many people torture i2c at longer distances to connect a sensor etc., this could make things easier and more robust if doable cheap enough. And will be a source of countless torn hairs for people who will inevitably push it to and beyond its limits.... :D .