I'm glad you managed to get the vet to call in Jess, it always helps for the vet to 'meet' a mare before foaling, to get a general picture and to have a chat with the owner.
I know you said the 'blood for coggins', but will he be checking it for any other possible 'problems' as in a higher than normal white blood count etc?
Also I'm a little suprised that he put down her discharge and milk running as probable labour indications when Darla has been showing these 'symptoms' since very shortly after you got her. It may well be that they are of no consequence, but Darla simply cannot have been showing indications of labour for that length of time?
May I suggest that first thing Monday morning you contact the vet's office again and ask them to hurry through the blood tests, then if they come back clear for any bacterial/viral infections, then perhaps testing a urine sample and taking a swab of the discharge might be the next step.
I am not suggesting that anything is wrong with Darla - her 'behaviour' over the last few weeks may just be how she is when in the latter stages of pregnancy, but for her sake I would be leaving no stone unturned in trying to find out what is causing her symptoms or alternatively, getting complete and totally satifactory 'results' to show that, how she is behaving, is completely normal for her.
If the vet's office now has Darla up on cam for occasional viewing, then maybe someone there will spot something 'unusual' in her behaviour and will offer some suggestions?
Really hoping that Darla produces her little baby soon and so gets some relief from her troubling symptoms, bless her.