Those 'squiggly lines' sure look like the tracks of some type of critter eating away at the leaves to me. You can buy different colored sticky traps and possibly see if you catch anything. Supposedly the brighter colors work best like yellow, pink, bright blue. Just a thought.

I can't see how it can be water splashing on the leaves. I've soaked mine before and had them laying in the dirt and they look fine.

As for fertilizer problems. The yellow 'tracks' I see aren't caused by any nutritional problem. The brown spots I have on mine right now and it turned out to be a pH problem more than anything. Water was to high. Lowered it down to around 6.7 and we'll see what happens. Just some thoughts. I for sure am not an expert. Just read a lot.