What are these spots? It's too late to fix, ~2weeks left. I just wanna know what the issue is to prevent it next time. TIA

I think you are correct in the assumption of salt build up. I think what you are seeing is a burn on the tips from too much salt and not enough water. Nick, from rooted leaf talked in depth about this and what happens is your plant is taking up salts (nutrients) constantly and sometimes you do not have enough water to go along with the salts in the tissue so they get an osmosis shock and it happens at the tips first because that if the furthest point from water. I assume that at one point within the last two weeks the soil got slightly too dry between watering and you are seeing the results of that. As far as flushing is concerned, I do not think you need to do a flush at the moment. What I would do is mix your nutrients to a slightly lower EC and feed more for the next two watering. And by more, I mean make enough mix to allow about 20-30% run off. This will rinse out a lot of your salt build up while still proving food for your plant. I hope this helps.