Parent Qty to Unreserve

There isn't a way to un issue based on the parent qty, but Un issue All option is available, there it is possible to un issue all the issued qty.

Can we create a request to unissue all parts based on un-completed qty?

Sure, that would be a worthy one to consider. There can be complication when it come to un issue in contrast to the Issue.

e.g. when the issued part is in an as built Structure of a multi level tracked parent part, which has been received to inventory.

Also if the issued qty exists in Shop Order Tracked Structure.

There is a risk if you un-issue qty based on the uncompleted qty - how would you capture any over or under use of materials if you assume that the process has used exactly what the structure defines.

Obviously this is vey dependent on the process but any calculation of actual yields or similar would not be possible in this scenario.