As with all training, specificity is the key. You have two challenges -
Running 6 consecutive days and recovery.
Terrain and climate.
Your training needs to reflect these as best you can. I'd suggest building up to running 3-4 days on the trot regularly, with an occasional (once a month or less) week of 6 days running. The structure of these days is really up to you, but try and keep some quality in there.
Not sure what the terrain on the Coastal Challenge is, but try to simulate this as best you can. Same with climate - many layers on a warm summers day maybe?
There's no shame in a DNF for any reason imo. You tried and weren't equal to the task on the day. The hard part is figuring out why, and then addressing those issues so that it doesn't happen the next time.
Looks to me like your actually racing too much - assuming you are going to be racing all your races. Where is the time to recover? and the time for your body to adapt to the training stress. Personally I wouldn't schedule another race between the quadzilla and SDW50, just make sure you're fully rested and recovered and get 6 weeks decent training done, without having to lose a week to recovery from yet another race.
After SDW50 you only have 4 weeks til TP100 - at least one week recovery, and at least 1 week taper only leaves two weeks 'training'. If you get caught up in the 'must race' mentality you risk burning out.