Does an entire journal entry get deleted, when only a single detail line was deleted?
a journal entry has to balance, credits and debits have to be equal, so yes, delete one of those and the entry can not exist
Journal entries should be the exception when using QB, they often do not work as you think they should, and when you use inventory type items they never work for inventory. It is much better to use the forms on the home page the way QB is designed to be used. Journal entries also bypass accrual/cash reporting, and will not show on many reports.