What do you mean by "a failed mist coat"?
From time to time, I will use acrylic primer when touching in filler that I have just sanded back over the "mist coat" on new plaster (and invariably sanded through the mist coat). In most cases I would still have some of the dilute emulsion left but when I don't, rather than diluting more emulsion, I just use full fat acrylic primer.
The term "mist coat" is rather contentious. Some people prefer to go for a 50/50 mix. I find that kind of ratio to be so thin that the paint splashes all over the place. I follow the manufacturers advice with regards to thinning the first coat- in the case of Dulux Trade paints, that is normally 10% water to 90% paint.
I am not a fan of using contract paint as a base coat, I have had issues when applying the final coats of emulsion in the past. If the plastering is going to require a shed load of sanding/filling/sanding/filling/sanding then I will often use dilute white Dulux Trade matt emulsion- largely because of the "relatively" low cost (compared to the mixed colours). If the plastering is to a high standard I will simply dulte the paint as per the manufacturers instructions- that said, I recently had the misfortune of using a Valspar paint which said that it should not be diluted. I used DT white matt as the dilute coat and then applied a full fat coat of the DT white matt to achieve a constant level of suction prior to applying the valspar.