Had you trained to match the race conditions? 11:30 is a late start, with your body expecting to shut down for sleep (assuming no time difference). Can be a very tough time to start/continue running, particularly if you're not used to it.
Sounds to me like you talked yourself out of it, and were relying on your pace group to carry you through - if you'd continued, you would most likely have come through the other side of your low spot - but only you can know for sure why you stopped.
Not sure I understand the connection to UTMB either - that's a world away from what you are doing currently. Take your time and build up slowly. Just qualifying for UTMB would see you undertaking some very difficult events.
Given your lack of training time, why not go for shorter off-road events until your circumstances improve to allow proper training?
Personally I go for almost full cover in white if it's going to be hot - normally a compression vest with a baggy t over the top, and white sleeves. I find the relectivity helps with feeling overly hot. I have a cap with the 'tail' - and like it when it's sunny - again it reflects the heat and prevents the feeling that you are being slowly baked
If you have time, try the compression socks in training - if not, save tehm for later in the race if you feel you need them (calf spasms etc.).
If you're getting too hot, probably worth reducing your effort level until you're comfortable. Contending with the heat, assuming you're going strainght there from here, will be difficult enough.