As I have understood, short interest rates vary. All shorts 26+M currently are paying whatever current rate from their brokerage.
There is a difference between short sellers stuck paying 16% or higher (this is the current Fidelity rate which historically has been among the lowest) because they do not want to cover their position at loss, which required may be 5% when they opened it and somebody entering the position at 16%,
with no likelihood of this percentage going down to sane levels before the acquisition vote.
Regardless of the above, big institutional holders will need shares to vote on the acquisition. There were no shares available to short at Fidelity as of yesterday, and because of the institutional holders trying to vote their shares, availability of shares to short them will not be there, particularly as we get closer to the vote.
BTW, couple of days ago I called Fidelity Trading Desk and asked about the acquisition vote and what Fidelity is planning to do to vote the shares that they lent. The response I got, after rep. put me on a prolonged hold to consult with the proper internal resources, that Fidelity will be securing the shares on the exchange to vote them. This is generic answer indicating that they do have procedure to get shares to vote them (this does not support low voting turnout theory, Brexit syndrome if you will). This also could mean that person consulting the rep. is not aware that locating shares on the exchange in the quantities necessary will be impossible without the recall, because in my estimation just 5 largest institutional holders (Fidelity, Baillie Gifford, Price T.Rowe, Vanguard and Bank of Montreal) will likely need to "locate" more than 22M shares, wich amounts to about 40% of all shares held by retail investors.
Incidentally, the form 13F data for Q2 are trickling in, and these largest holders
were increasing their positions, including Fidelity (they mentioned by 9%), Baillie Gifford (by 9.54%, or 1.14M shares), Vanguard (by 13.4%, or 0.58M shares), Bank of Montreal (by 6.8%, or 0.31M shares).