most cases will provide you with the grip. At Machines store, you can try on the cases before buying it. I always do that

I think it's quite hard for you to type using 1 hand if you're using 6+. No issue here with one handed typing with iphone 6 though

As a 6+ user, i would say definetly need to use with a case as the phone itself is quite slippery (same for 6 as well but you dont notice it so much as it is smaller and easier to grip), as for the one hand typing, it still comes down to the individual user's hands size. for me the buttons on the extreme left side of the phone is quite difficult to reach when using one handed.. quite a stretch on the thumb (im right handed).. but the plus point of the screen size and battery life overrides this one problem