Try Nouveau, like Brickfrog said, if only to test the gpu.
I dont know if its the same but in Mint the Synaptic Package manager shows me all the versions of the nvidia driver supported by the kernel. Maybe you can downgrade your driver with it, although, I dont know how would you do that. Im just guessing but I think you would need to switch the transitional packages and those pull all the packages related to the driver.
Also, I've read a few times that distro upgrades can have weird problems and clean installs are better.
I dont really know how to use Inkspace so my method is probably very roundaboutish but the way I used to do it was:
1.- Use gimp to turn the image into solid colours. IIRC the option is called posterize, I would choose 5 coulours for this Image but you have to experiment whats best.2.-With the colour selection tool, select one colour and make a new png for only that colour shape.3.- Do the same for the rest of the colours.4.- Import one of the colour pngs into inkscape and use the vectorize function, I very rarely needed to do adjustments to it unless the shape was complicated.5.- Remove the imported png and adjust the colour of your new vector object if needed.6.- Repeat 4 and 5 for the rest of the pngs on the same inkscape document.7.- Align objects if needed and save to svg.
Hope I explained it in an understandable way, if not, feel free to ask.