1.

After completing the first international segment you can change routing. As long as you don't change the continents this will cost USD125 + any service fee that may be applied.

Inventory does not equal itinerary. Inventory in this context represents the availability in the booking class of the type of xONEx you purchase. In the case of xONEx's it's basically A, D and L. This is important, see 3 below.

2.

Absolutely valid. Only limitation is 4 segments in each continent except for Nth America where the limit is 6.

3. You can book open segments with no issue. However you do need to actually book a flight before you travel. The inventory class relating to your booking needs to to be available at the time of doing so. The closer to the time of travel you do this the less chance of it happening. Space available does not come into it!