The design looks very good, especially with thoughts of lowering at harvest. You are right in your thiking that they'll tangle into a mess at the top. You'll need more height after this year.
When you say that they're not doing so good, what do you mean? They're not up yet, or they're up but not growing? Once they break ground and taste some sun and water it'll be on! I have plants of varying ages around here. I have stuff three feet up the rope to a new planting that has yet to break ground. I know that SoCal is quite a bit different from here weather-wise, but be patient.