Playing solo "Flies and Spiders"
If you're playing solo, you count as the first player at all times. (You also count as the "last player" for any cards that use that term, but if any cards talk about the "next player" then there is no next player, so that card's effect would not be able to resolve).
Therefore, when you turn over stage 2B, you'd follow its instructions like this: reveal stage 3 and create a separate staging area for you to use in resolving stage 3. Then discard the current staging area and all its cards, so that the stage-3 staging area becomes the only one. Then at the end of stage 3, proceed to stage 4 as normal.
In other words, when playing solo, you will go through stage 3 as normal after doing stage 2. The only difference is that in a multiplayer game, the other players are in stage 4, fighting spiders while waiting for Bilbo Baggins to show up (because until Bilbo shows up, there's no way to explore The Spider's Glade -- and you have to explore it to win stage 4).