It's so hard to say but:
I don't put much stock in the claim made by IAMS that the investigator authorized the tests as it is no more credible than the a dope dealers claim that a 'cop smoked the dope too' while undercover. In other words she did what they were already doing in order to investigate. If she had the authority to do that in the short time she was there then it would seem odd nobody stopped her if it was against their policies
. It is far more likely she continued doing what they were doing already.
Why nobody else but PETA is complaining may be because of the money. IAMS has made some large contributions to the Humane Societies so once again follow the money trail. Same with the TV stations (Protector and Gamble = every fifth commercial we probably see). Also remember that animal testing is not illegal - just ask Tim Dack.
If they have changed as stated then good. Still look at the product range of Protector and Gamble and then consider that they probably test on animals for other reasons. There are so many better foods to feed anyways from companies that have never done this - not once, not ever.
I too think that PETA often take things way to far (I do eat meat and don't support BSL), so much that they may actually harm their own cause but they have done some very credible work over the years.
Finally: As far as the HSUS statements go it's not really an absolution of IAMS it's a statement that they would rather work with not against IAMS to facilitate change:
The HSUS has neither endorsed the past actions of Iams, nor the company's current practices. We are, however, working with the company to see that any animals under its care are properly treated. What's more, we are demanding that the company move away from laboratory tests that might cause harm to the dogs and cats, and put their resources into clinical studies that result in no harm.