I think it'll rise faster than 18% for some time starting soon, but then settle down below that, so that Summer/Autumn will have higher difficulties but 2014 Spring could have lower ones than what's in that table. But as said, it's just guesses...
This is assuming an 18% increase in difficulty each checkpoint. Which I understand is about every 2 weeks??
Almost, but not quite - if the difficulty increases by 18%, then the time to reach that change will be 18% less than 2 weeks. (because the 2016 blocks were found 18% faster).
Close, but it's actually 1/1.18 ~ 0.85 or 15% less than 2 weeks. 100% increase would be 50% less, or 1 week etc.