September +/- 1 month. It will take you around 2 months to get full cp cata gear aswell, so you're looking at around 1 month of playtime before the expansion is released. I wish blizzard would split the seasons in half and just reduce the price on gear in the later season so one didn't have to play vs people with +10 ilvls on a new character for such an extended period of time.
Ya, it's not worth the grind at this point. Everyone is prepping for mop. You can grind some honor for weapons to transmog if you like the art, but it won't be worthwhile if you're trying to be competitive. Mop will be out a week after you get full gear lol
I would've said MoP release on August/September. I don't see it being released as late as October unless they go with a November release but that wouldn't go down well with the FANZ. Season will end a bit before MoP pre-patch which will be a couple of weeks before MoP release. Early August/September imo.