Shorter:
Assumes that the swap rate will be relatively constant for the lifetime of the contract.
Assumes that the value of the contract will decrease over the lifetime of the contract due to mining.
Has no power to ensure that either assumption comes true.
Manipulator:
Assumes the swap rate will increase each time the swaps are renewed. Ensures this happens by maintaining a floor price (thus increasing the contract's premium compared to its mining value and encouraging shorters who will compete and be willing to pay for the increased swap rate).
Assumes the contract will maintain its value until Nov 15 (the renewal of the last round of 30 day swaps). Ensures that this happens by maintaining a floor price using their own funds.
Result:
Shorter - makes a trade that looks slightly profitable given difficulty expectations.
Manipulator - makes a trade that appears to lose money given difficulty expectations, but will make a large profit once they manipulate both the contract price and the swap rate. Has almost all of the power to affect the swap value and determine the loss or profitability of going short or long.
That only works if the manipulator is the only person offering swaps. If there is competition, then the manipulator that tries to set a floor risks losing money on TH1 with no swap to compensate.
I believe there is no manipulation going on. Instead, there is a shortage of swaps because some TH1 longs are simply holding TH1 (and losing money), and some TH1 shorts are paying higher interest rates than they have to.