After my post of these issues I've - again - got a reply from Team Durango within 24 hours. They redirected me to the customer service from Hobbico and I got my parts within 4 working days and received replacement outdrives, driveshafts and bearings (for the ones that cracked on assembly).
It seemed this time around the package of the outdrives had been opened - perhaps to check if the bearings would fit on. The driveshafts also seem ok looking through the packaging material
Upon rebuilding the diff, the outer bearings did fit on properly this time. However the female side (in which the thrust bearing and 5x8x2.5mm bearing is inserted) was so tight the parts didn't fit in. I tried another thrust bearing and had exactly the same problem - as was also the case when trying to fit another ball bearing. I had to heat up the part to fit the components in there. I don't know if the tight fit will cause problems with wear of the bearings, but it sure will be a pain if (or when) the thrust bearing needs maintainance. I guess time will tell what the severity of the problem is, and whether the wear of the outdrives has been solved.
Lastly, I noticed the hole in which the driveshaft knuckle/head fits is so big there is over 0.5mm of play in there - it doesn't seem to affect the function so far, but the fact that the car sounds louder with new outdrives is not reassuring me so far that these new outdrives are better than the old ones - but again, time will tell: conclusions can be drawn in the long term in this case, not with a first impression!