I cannot see how any DG could distort an investigation and be confident that not one of the standing army of civilian investigators that are not under his control would fail to mention it to anyone!
I highly recommend reading the evidence to Lord Phillip. One of the AAIB investigators revealed he had not been shown relevant evidence, prompting him to seek a meeting with Phillip. I agree the AAIB are superb and so too a co-located MAAIB but investigations are only as good as the evidence they obtain or are granted access to.