1. Agate Clown Indra is probably the best place to train. If you are feeling confident, the grand hall trainers are your go to but they have mons 80+
2. You need to get to the 8th floor of the obsidia department store, additionally you can buy EV erasers in the new shopping center at peridot
3. Best way to EV train is to use the Power items and kill lots of pokemon under the railnet. If you don't need speed on that pokemon (noibats and swoobats give you 1 speed EV) , then you can go buy another speed eraser
4. There are plenty of walkthroughs on how to get beldum to replace your noivern. Also there is an online trade system now which you can use with other trainers. Honestly EV training will make your team considerably stronger and you should be able to beat the gym with your present team