If I look at your schedule, it would be much cheaper to take the domestic flight from Amsterdam to London (
www.skyscanner.net) (I am not too sure about the fares in May/June). But this website shows you all available airlines and fares, and you can purchase the tickets here.
London to Paris, you can choose either take the Eurostar or domestic flights, but be aware of London have 4 airports, if you take the domestic flight, make sure you get to the right airport, most of the domestic flights are not flying from Heathrow Airport. But this route from London to Paris, I think would be cheaper to take the train, as from the city to the airport always take times and fares are never cheap, and the train stations in Europe are always just located right at the city centre.
From Paris to Holland, you need to take TGV, even though you have got the Eruorail pass, you still need to pay some additional cost to TGV, so, if your schedule is fixed, you could get some price information at 學聯 for a Eurorail pass and also for two single tickets price, see which one is cheaper.