Can I enter "zero" as an adjusted cost basis for sale of interest in a partnership when the cumulative adjustment to basis is more negative than the original cost ?

That would be what I recommend.  However the facts are not clear on how to handle the other aspect of this disposition:

• Technically your basis cannot go negative.
• Based on bullet number 1, do you have suspended losses?
• If you do have suspended losses, you can only take the losses to the extent of your basis; effectively arriving at a zero basis.  Anything remaining will be lost; no economic loss.
• If you don't have suspended losses you will need to pick up the "negative" basis amount as capital gain on Schedule D and the applicable form 8949.