I'll try and put some feedback here for the NZ/ commonwealth model - hopefully anyone will correct me if I go wrong.
Q: In which countries is it usual to have a detachment of the field unit at field depots to organise, reserve and prepare supplies for the truck convoys of their field units?
At the rifle company level the CQMS (Company Quatermaster - an infantry SSgt who acts as the under-study of the CSM while acting as the chief point of contact for all stores stuff) is often allocated veh/ convoys to organise and arrange for resup. Doctrinally they are supposed to stay with the F Ech (the coy) but my experience in FOBs and on exercises is that they practise a high degree of mobility, and often push themselves back to the A Ech when organising supply.
Q: How 'normal' are convoy security units internationally?
As I understand some countries assign scouts, AT Plts and others in makeshift solutions for convoy security.
That may work half-satisfactorily in COIN, but what's about great, mobile wars?
I'm not going to swear this is true, but I have spoken to both US and NZ MP's and they both believe it is their doctrinally enshrined right to escort convoys and control/ provide route security.
Now, what great insights and commentary have you got on this topic Fuchs?