M-Edge ones are often sold as non-specialised. I have two of them, one that I use with my 505 but became the "fitting" for the 600, and the new one that goes with the 600. I can say they're different, even being of the same line and the same builder, the first being a perfect fit but in green- my 600 is red- while the second, in mocha, could have the button issue you mention.
One solution is to cut a bit of the strap (it's feasible and not too defacing). I just found another solution: I used a bit of the plastic foam the cover comes with, cutting just a 2 cm x 0,5 cm rectangle, and inserted it between the strap and the button, and it works- it cushions the pressure. I hope it will change with time; anyway the plastic foam is not too visible (dark grey on mocha, and there's a light grey microfibre lining inside the cover).
As for the light, I have a clip-on light (which works better on a Reader/liseuse than on a book, it won't go in the way for the page-turning
) and I'm waiting for a Mighty Bright reading light. I'm not really sure about the light wedge- a lot of people complained about it in the 505 forum, and of course there's the issue of the clarity of the 600 screen, so one wouldn't feel so sure to recommend it! (the light-wedge one for the 505 isn't perfectly ok for the 600, also.)