We enjoyed the Grand Floridian Cafe as another option for you. Keep trying for Chef Mickey's or Ohana; you might get lucky. We weren't able to get Ohana at first, but got it closer to our trip. Your dinner time will be more tricky though.
I would give yourself atleast 15 minutes, but probably more for the monorail trip. Sometimes you can catch one right away, but we had lengthy waits for the monorail several times (of course when we were in a hurry and needed to get to an ADR) and once it took over 20 minutes for one to get to us. We were late to our ADR but luckily we were seated right away. If for some reason the monorails are not working you can catch a boat to Magic Kindom and walk (I have read it's about a 5 minute walk) or take a boat to CR.
You should be more than OK with your 6pm ressie at Kona. Even if it took you 2 hours to eat, which I'm sure it won't, you'd make it to CR in plenty of time.
Best advice I can give you on getting done at a restaurant: settle your check as early as possible, especially if you are on a dining plan and tell your waiter ahead of time that you need to make it to CR by 8:45. You may want to order your appetizer, entree and dessert at one time to save a little time as well. Anything can help, especially if you get something with a later ADR.