I'm not a big fan of the two fonts that were chosen. I think you should choose one easy to read font and stick with it. I also think the color of the text within the drop downs should be a little bit lighter of a blue to be more legible.
Add To Cart
That button should be about twice the size, showing users where exactly they need to go would be a good idea.
The slider in the bottom left hand size that you mention changes the canvas orientation, this has no label so users will have no idea what it does until they click it. Everything should be labeled.
I like v2
I like the drop downs to be labeled product, size, framing, personalize, I think that's more descriptive and user friendly.
I really like the icons used in the corners of the canvas area, I think they are easy to view and should be an easy size for users.
Overall I'm actually surprised how simple and aesthetically pleasing this design is. Most customizable solutions like this are quite clunky and usually pretty dull visually so I applaud you. I wouldn't make any major alterations besides perhaps inverting the colors and make the background white with black text, this is a lot easier to read and view for users, plus white background websites typically have higher conversion rates.