I've switched a few mags in difference PU with mixed results. It's an easy swap with PU's that don't have covers. One thing to watch out for is the orientation of the mag to keep the PU in Phase.
I love the those little G10F-15's I find them very balanced, they are a green back without the green plastic covers. I switched out the G10N in my JMP1 for one, have another in a JCM1 and JTM1 OFFSET and one in a Class 5 cab. I use them with all my amps, thought about making a 4 x 10 with them, I ke...
The DSL40C is a very versatile combo for the price you can upgrade the tubes and speaker and even buy the extension cab and upgrade it's speaker too and still be under your 1k.
If the amp has the 4 ohm tap then no problem, you may feel/hear a difference in the response of the amp, you can try and run the cabs in series (16 ohms) by making your own cable.
http://images.gibson.com.s3.amazonaws.com/Products/Electric-Guitars/SG/USA/SG-Standard-24/Gallery-Images/SG24MEBCH1-Finish-Shot.jpg I put a Dirty Finger in the bridge of my SG 24 Standard and put a solid chrome cover on it. http://www.gibson.com/Products/Electric-Guitars/SG/Gibson-USA/SG-Standard-2...
They should be equal, 16 ohm cab set head to 16 ohms, 8 ohm cab set head to 8 ohms, if you run 2 x 16 ohm cabs which in parallel is 8 ohms so set head to 8 ohms.
If the cab has the switch in the back for 8 ohm stereo or 16 ohm mono, set it to 16 ohms
Question 1 is a yes, just as you stated, number 2 will work up to a point, the G12C are only rated at 25 watts, so if you go too loud with the 100 watt VM you could blow them.
I was looking at the guts of my C5H, boy that R51 is a little sucker, kind of tough to get at. I saw 2 unused male tabs by the headphone jack labeled speaker + & - do you know what they are for ? I guess I'm going to just put some clips on them and connect it to a speaker and see, hope it's the same...