never had a staghorn fern, but if it is a epiphytic its roots are more for fixation than for nutrient absorption, so it should be fine, but a more experienced person would give a much better answer.

First of all, *thumbs up* for the Better Gro sphagnum moss you're using. Great stuff! Secondly, just give it some time. It may not look its best with the fishing line exposed, but it'll grow new shield fronds to cover them back up in no time. Just keep caring for it like you have been, and it'll take care of the rest. The damage incurred was merely cosmetic and not in any way detrimental to the well being of your fern.

There's no definite answer. Do you move it outdoors after the weather warms up? When I mounted mine, it didn't have any shield fronds whatsoever. It wasn't until the end of summer before it began to send some out. And I had mine growing outdoors for most of the year, where I hosed it down quite frequently. The more (indirect) sunlight that you can give it, the more energy it'll be able to photosynthesize and utilize towards sending out new growth.

Better Gro also sells an orchid fertilizer that I love using for all of my epiphytes. I would highly advise you to get some if you haven't already. My local Wal-Mart sells it every year. If yours doesn't have it, then you can order it online and have it sent to the store for free pickup. Lowe's sells the Better Gro brand, too.