As I see it, this can't be done (as you would like) on CO/NW for a number of reasons, but there are alternatives!!!

You have booked HP EWR-SAN and SAN-FLL. If HP published fares FLL-EWR, you would be good to go on the FLL-EWR leg to complete your circle trip using half a discounted RT fare, but alas, HP doesn't fly or publish 'codeshare fares' which would be valid on this route... therefore, no completion of a circle FARED trip for you... again, using half a discounted RT fare on the FLL-EWR portion.

Best you can do is to use a OW fare. And unfortunately, the cheapest OW fare on NW/CO is the $589 that you mentioned.

I suggest that you fly FLL-EWR via JI. They have a OW fare of $140 something and you can fly via RDU. (If you can get a seat).

As a consolation for your losing this one segment on CO/NW, I would suggest that you re-book the HP flights as follows - as this will allow you to pick up one more segment for the one the you will lose.

I believe that your HP flights came out to $ 428.50.

The HP 'mileage run make-over' suggestion below comes out to $313.50 - because these flights are booked SAN-FLL in the cheaper 'L' class... whereas some of your selection was booked in 'W'.

1. $HP 38 L 24MAR SU EWR PHX 548P 934P 319 S 0
2. $HP 192 L 24MAR SU PHX SAN 1025P 1035P 320 0
3. $HP 14 L 27MAR WE SAN PHX 935A 1149A 320 0
4. $HP 273 L 27MAR WE PHX IAH 1251P 428P 733 0
5. $HP 1748 L 27MAR WE IAH FLL 710P 1032P 757 S 0
HP 1748 CONTINENTAL

Tvl4free noted that, once again, you didn't mention a date, specifically for the final leg(s).

Here is something which showed available.
Total cost w garbage of HP and JI is $466.

6. $JI 122 V 29MAR FR FLL RDU 240P 440P 737 0
7. $JI 328 V 29MAR FR RDU EWR 540P 725P 737 0

You've done quite well with this itinerary and tvl4free is quite impressed. Chosing the HP flights was a good choice, as HP is the only carrier on the EWR-SAN portion which has a fare that is usable without a Sat. nite stay. If you use any other carrier on the EWR-SAN-FLL portion, the fare becomes astronomical.

However, if you really would like to get aggressive with this itinerary (and tvl4free is always very aggressive with itineraries) , you could buy a tkt FLL-RDU for about $ 59+ on JI (just to get outta town)... and then another RDU-EWR on CO for $59... and pick up another CO segment... RDU-EWR...

Oh my goodness, look what we've done... and tvl4free is all excited now... and so I'll have to stop for today!

All in all, yummy, yummy!