A couple of things to add:
- I updated the wiki to reflect the different behaviours for LTI 1.1 and 1.3. I hope that it is clear now.
- I don't recommend reverting to LTI 1.1. It is considered deprecated, and at some point Desire2Learn (and other commercial learning management systems) will stop supporting it.
- At our institution we do what Alex suggested: hide grade items from students in D2L until they are complete.
- In D2L you can configure how blank scores are treated. If you have "Treat ungraded items as 0" selected, then you don't need to manually fill in 0 for blank scores. Unfortunately this setting is at the course level, so you can't do it for just WeBWorK assignments.