There is so much mis- and out-of-context information online. This tends to make thinks more complicated than necessary.
Yes, over-watering is a common problem with potted plants, but that is usually caused because people tend to put plants in pots that are larger than necessary. This is not something you need to be so concerned about. The leaf spots were probably caused by soil that got too dry. Those spots are permanent and will not go away. If that bothers you, trim them off. A thorough watering about once per week is about right for your Fiddle-leafed Fig if you keep it close to a sunny window.
This plant does not require high humidity and misting is not an effective way to increase humidity anyway.
Chlorine in tap water is not a problem with potted plant. Allowing the water to sit overnight doesn't help anyway.
Cold drafts are not a problem at this time of year. In fact, good air circulation is a plus.
Soil salts will not be a problem as long as you don't over fertilize and you water thoroughly when you do water.
If you are concerned about the plant's lean, simply pull it up a bit from its pot, reposition it slightly so that it is upright, and then tamp the soil down to hold it in place,
Finally, Ficus lyratas are slow growers that often go several months or more without putting out new folage. Be patient. And be careful of information you find online!