I did some research and found some of my own answers (be they right or wrong) and have a further question and replaced Q2.
Q1) "LB" is Line Box and it is the junction box I suspected. Further question: do LB have to point down or can they be directed in any direction? Do they have to be a certain distance above the ground?
Q2) Replacement question: For THHN in conduit the 8 AWG will handle 55 AMPS, the spa installation instructions also recommended switching from 6 AWG to 8AWG for a short distance going into the spa. I'm planning on 8 AWG Red, Black, White and Green in the conduit.
Q3) Not going to use unsheathed Romex
Q4) I'll put the disconnect at 48" above the deck.