Compulsory purchase - Heathrow is offering 25% above market rate and has a large allocation set aside in its costings. Should a commercial agreement not be achieved then yes, the state can compulsorily acquire it but would pass the cost straight on to the developer. This is what page 328 says.
AC forecasts - if the AC's methodology is flawed and they didn't predict LGW's growth, then the same must be true of LHR's growth. In which case, it's worse than first thought. The government is artificially influencing the market by preventing LHR from expanding and driving growth to LGW as there's no alternative. That sounds like State Aid in favour of GIP.