My 300tdi is similar. Cold mornings/first start for the day it turns over a bit slow but still starts within 2seconds. Some mornings the coolant temp is around 3 deg, t-case temp sender around 1 degree. I think by memory my starter battery is around 800 cca. Sometimes I double glow, still seems sluggish.
I have just ordered a new set of glow plugs, was hoping that might help, but after reading replies on here doesn't sound like they do much anyway... They are well overdue to be changed either way, over 75,000kms ago. After reading some horror stories on here as well about the tips falling off into the cylinders I am worried about them as well.
Maybe it is the earthing or the starter on the way out?
Regarding mixing petrol into diesel, I just found this link, bit of a read but
basically its a no-no. Yes it will thin diesel out and aid in cold starts, but it takes away lubricating properties, will increase EGT's, and anything with a high octane rating will retard diesel fuel’s ability to ignite. I add Diesel Power additive to every fill, don't know if it does anything but haven't had water or other fuel dramas since I have had car.
Mixing Gasoline And Diesel