I don't use any external boxes, but these are my best understanding.
1) Yes, but with options.
But you may be able to blast a power-on command before each channel change command. But that would be a bit complex and I'm not sure it would work reliably. You would be better to look into devices that are able to turn on other electrical devices when a PC becomes active. I believe that there are powerboards now that can power one or more outlets all the time, for the PC, and have a USB input to trigger turning on additional outlets. That way, when the PC is on, the STBs are on.
You will need to do this if you wish to record programs from the STBs, and MC will need to wake the PC (if you are letting it sleep of course). When the PC wakes you would set it up to turn on the STB outlets. This would require a little experimentation, as the STB needs to be awake and functional before MC sends the channel change command to change to the channel that needs to be recorded. So there could be a timing issue. Probably solvable.
Also, most people leave their STBs in standby mode. Whether a STB will fully power up and change channel from standby when it receives a channel change command is a function of the STB design.
2) Yes, if I understand what you mean.
You could set up separate areas in MC for watching each STB, in both Standard and Theatre Views, for example for 'Watch CableTV' and 'Watch SatelliteTV'. That would just require customisation of the menus and navigation pane using appropriate rules, which is a standard capability. As you used the term "Skin", I thought at first that you would want different colours and components for each STB. You won't get that. Also, when watching Live TV you would just select the appropriate channel using Channel Up/Down or the Channel Number on your remote. All channels are available using that process.
3) Yes and Yes.
You can import EPG data from as many sources as you want. You need to manage it properly if there is overlap between the sources. Some sources are built into MC, but any other that you want to use can be incorporated using the XMLTV import function. It can all be automated of course.
You can set up as many EPG Guides in MC as you want in Theatre View. It is done just like setting up menus, but the EPG View has a special layout to produce the typical Gantt Chart display used by EPGs.
Standard View has just one EPG "Guide", which is more a listing of all channels and programs they carry. However, you could set up views in Standard View to show separate sections of the EPG, if you wished. I can't think of a way to make those Views active though. They would be for viewing and searching only.
Ultimately though, for 2) and 3) above, MC works fine with all the channels from all sources presented together. There is no need to separate them. You will see that as you begin to learn MC.
While I am here, I do suggest that you spend some time reading the Wiki if you haven't already. There is a lot of information there.
https://wiki.jriver.com/index.php/Getting_Startedhttps://wiki.jriver.com/index.php/Category:Frequently_Asked_Questionshttps://wiki.jriver.com/index.php/Media_Centerhttps://wiki.jriver.com/index.php/Main_Page