I may have to pay DDs to wax this lot Very funny with your name!! but yes, I'd be tempted 
No, I haven't/don't get lumpy paint. It shouldn't be lumpy, not at all. It should be 'silky and smooth'. I wonder what their turn over of paint is & what their storage conditions are like??? Though, it should keep for a long time unopened, it's just funny you are both having the same problem with paint from the same place.
Did you turn the tin upside down for at least half and hour then mix it for aaaaaaaaaaagggggeeeeeessssss?
Adding water is fine, but do it very slowly, just a tiny bit at a time as it's easy to make it too thin, then it runs. I do it with a sports water bottle. ... and yes GG you can make it too thin, then it will rub easily, but keep in mind that it is a chalk paint, so it does rub off, sort of.
I'm going to be down your way in a couple of weeks, maybe we should meet at T&L then go for coffee/lunch?!