Yes and Yes - that's the theory. I did have an instance recently after adding some Armor and I set the durability too low, the game recognized that the armor needed repairing but it wouldn't repair it. However, a few moments later, it would. I'm not sure what the trigger was but I've certainly seen situations where the game doesn't acknowledge certain things immediately after loading some newly added items. An example, adding 99, say, lightning resin - they would be in the inventory and they could be used from a hotkey but the game wouldn't interact with them using the Item menu i.e. "Use Item" or "Drop Item" - then a minute or so later, it would.
I've been leaving the Max Durability as 0 without any problems (jappi is really the only one who'd be able to tell you exactly what is going on with that field).
The only bits of kit that I've seen with really low durability are the Crystal items. For example, an Uchi normally has a max durability of 80 but the Crystal version has a max durability of 8. It would appear as though the max durability for the Crystal items is 1/10 of the other flavours of the same item.
It was most likely due to setting the Durability to 9999. Another issue is that it will not allow me to Drop any armor/weapons I've added with the editor