A small gap in the top ceramic is normal. Allows oil to flow down to the heater core and the wafer underneath it.
The gap is also what lets air flow past the plate. Without it oil seals the the plate, the draw gets harder and your head implodes. Early units had two small gaps (opposite sides) but they could foul out easily so they shifted to one larger one.
Taste change is often an indication that you've run out of ready concentrate to hold temperatures down and they have rocketed up and you're now burning rather than making vapor. You need a fair bit, say .10 grams or so as a guess, to 'prime' the ceramic. You might get a hit or two from it with a new cart, might not. Then add another bit (.050 grams is a good target to start). Even when flooded, they recover well. Don't be shy about exploring fill levels and heat. Get it right and it'll do what Cera does for a hit or two. Even with casual use you can get half a dozen or so very impressive hits between top ups (of about 1/20 gram each).
I wouldn't panic about window air leaks right off either. Once it gets some oil deposit on the outside of the inner tube it'll seal just fine with the window 180 degrees out. Most of us probably pull the inner sleeve out and top load rather than use the window. You do have to wipe the outside from time to time if oil gets there, but not really a big deal.
A cracked plate is not fatal, as long as the pieces don't fall out (and even if they do, it'll run without the top plate at all). I'm not sure I'd rush off and have it replaced, for sure the gap issue is not a fault but a carefully controlled feature.
Good luck with it, it has a well earned reputation. I suggest giving it a chance?
OF