Yup, that sounds like scale. Aspleniums (including bird's nest ferns as well as various others) are very sensitive to it, and I haven't figured out any feasible way to keep them under control. Apart from toxicity problems, pesticides don't seem particularly effective. Picking them off by hand can work if you do if often and thoroughly. I don't know about staghorn ferns, but ficus certainly can get bad scale infestations. Also, for what it's worth, the sori (clusterings of spore-producing sporangia) on a bird's nest fern would look like straight dark lines starting near the center of the leaf and extending about halfway to the leaf margin. Here's a picture I took of the underside of a leaf of a large bird's nest fern with sori: Patrick Alexander

I agree, those brown lines are spores. Your problems start from the fact that this fern is an epiphyte or a lithophyte. In other words they grow on trees or on rocks. The most covering the roots usually get is from the moss growing around them. In the rainforest type of environment they naturally grow in there's usually high humidity with frequent rain and water never sits long around their roots. Hence, as Rhizo has stated, the signs of chronic over watering. You need to treat it as an epiphyte.

Well, the (possibly) bad news is that it isn't an asplenium. If you look at the stems and compare against asplenium pics, you'll see a totally different leaf shape. Probably someone jammed in a tag that was lying on a shelf without regard to what the plant actually was. The (hopefully) good news is that it looks like a peace lily (spathiphyllum), which are better at coming back from extreme heat than aspleniums. It sounds like you've done everything right so far. In fact, if you wanted to submerge the pot in a sink full of lukewarm water for an hour, that might accelerate its recovery. You'll probably lose some more leaves due to shock, but that's normal. Peace lilies are semi-aquatic and very difficult to over-water. You'll notice the leaves unfurl from the various stems in the general clump, rather than each new leaf growing out of the center of an asplenium's roseate. Avoid placing in direct sunlight. Leaves turn yellow with too much light. Don't fertilize for at least a few weeks -- that would be too much shock for the roots. Once it's healthy again, you can fertilize to make it bloom.

In a planter without a drainage hole " a layer of porous lava rocks lining the bottom" DOES NOT ensure proper drainage. Only porous soil and drainage holes ensure proper drainage. You need to get a pot with drainage holes.