It's hard to even tell that it's a ficus Lyrata.

How often had you been watering? What type of soil is it in?

I am staying in a hot and humid tropical area all year round, I water it once a week but just briefly around the soil. Unfortunately it is all bald now and it looks like its beyond saving..

My other plant B have brown spottings all over and I've attached a picture of the root and soil, hoping to get some advice on what happened to this Plant B and what should I do to save it?

Seems like your plant need repoting since months ago. It's dry. Your watering weekly seems too little and not reach living hair roots down there. Give water thoroughly, till it flow from bottom of the pot.

It is a rainy season here although my FLT is planted in doors, I am worried that the soil won't have time to dry out and the root will be rotten.

Under normal metabolism, the plant will lift excess water up and good air circulation will move it away. Transpiration and guttation. Most of vascular plants have guttation ability to remove excess water. Keep your weekly watering, just give it water thoroughly, not just the surface. Even in rainy season, you should keep watering your indoor plant. Plants uptake nutrients in soil only if water present in there, so soil watering is a must.