When plants are relocated, their environment is altered and so are their watering requirements. Increased light, temperature and air circulation will all contribute to a plant's needing more watering. It is also common for older leaves to die back some in response to a changing environment. You seem to be mindful of those changes and are making the appropriate adjustments.
Your plant most definitely does NOT need repotting. Plants do best when moderately potbound. That means when the outer rootball is almost entirely covered with a webbing of roots. As long as a plant has enough soil to go several day or more between thorough waterings, then it does not need a larger pot.
Misting does not improve humidity for more than about 15 minutes out of a 24 hour day. Marantas will do fine in low humidity as long as the soil is kept properly moist. If you want to raise humidity, use a pebble tray or a humidifier.
