I really love the map. I kinda think that the justification for it all is over thinking it a bit. These could be historical webs or it could be from a non spider. The party could be preparing for spiders and something else pops out. Maybe its like a slug thing that eats fungi and spins webs. In D&D you had the 2nd level spell web so maybe these could have magic origin from drow or cave sprite or something like that too. So id say keep the webs and as a GM get a bit creative with what might have caused them.

My scout will get out his flask of oil, flint and steel...