"1) I can't get KPV3 to display anything on the simulated tablet or phone "screens" in the preview window. All I see is the frame of the selected device (tablet, phone or kindle reader), with frame-less pages shown on the left in the preview window. I've installed KPV3 on my wife's win7 Home PC and there previews work showing the previews in the selected "device", although that PC also has 4GB Ram."
I have Oracle Virtual Box(OVB) running both on my Linux Mint and Windows 10 system on my laptop without any problems. To run the OVB emulator I also have 12 GB ram on my laptop. Quite frankly I would also have to say that running KP3 within QEMU with only 4GB ram is rather laughable in terms of the memory that's actually needed for that emulation. The KP3 application alone is about 1GB in size!! And I would never even consider using any emulator unless I had at least 8GB ram available on my computer.
"3) KPV3 dislikes floats on both installations I tried. If I have a float like a small image or a pull-quote set to any width less than 100%, KPV3 stretches it all the way across."
I don't understand when you say your using "floats" and "pull-quote" to position your images and text. If you're just talking about size and position of images, for epubs I generally use % values in my <img> tags which specifies the actual image size in terms of % of current screen size. Like this:
<p class="centered"><img alt="img001.jpg" src="../Images/image001.jpg" style="width: 50%;height: auto;"/></p>
In epubs you only have the choice of left aligned or centered for images. And you should not be anchoring images surrounded by text in reflowable ebooks. So forget about using "float" for positioning in epubs. You're asking for trouble if you do that. Centered or left aligned is really your only sensible choice for images or text.