You ask a good question narmac.  "why else would you short a play that could explode in your face"  

The warrants that trade came into being in June of 2017, and after they hit the market short interest shot up to over 800K but came down very quickly thereafter and then hovered around the 170 to 200K area until the more recent spike which seems to have been brought on by the, (now withdrawn) proposed share offering in April.

Maybe the shorting is a hedge or some other instrument....but frankly I think its simply that some big houses overplayed their hand when it appeared there was going to be a share offering, that's my opinion.

As to why they simply don't cover off....I'll suggest an answer by asking another question.  

Do you have a limit order in to sell any shares?  I don't, and I suspect most of us posting here don't.  You can't buy something unless someone else is willing to sell it.  And if there are shares available for sale, but at prices north of $3 say....

What do you think might happen if RVX all of a sudden jumped up from $2.40 to above $3 over the course of just a few days or even a couple of weeks?  

I suspect that if RVX suddenly spiked up to $3+ that it could lead to a wave of buyers moving in. And as we've seen from all the delivery failures, it looks like MMs are already having trouble locating shares to meet current demand.  

I believe (this is opinion only) that those behind the short interest....that they very well may be highly interested in covering, but to do that there have to be shareholders willing to let go of their shares.  Up to June 30th the short number is almost 1.3 million.  I don't know if there are that many genuine shares (not including shares on offer that haven't been located or borrowed yet) available.