(1) quite possibly - that the Newark flight to Heathrow was delayed due to a reason that would exempt compensation for that flight, does not mean that the weather issue delaying that flight can be used to deny compensation for the LHR-EWR flight
(2) The airline is liable up to XDR1133 (Approx GBP1250) for lost/delayed baggage. If you need to make purchases you can just do so and claim against the airline. It cannot decide that the Montreal Convention does not apply to it and create its own limits. It is up to you where you make the claim - if the amount that you need is within the AMEX limit, I would claim off of AMEX; if your claim is going to exceed that which AMEX will pay, then I would just claim directly off of the airline
What counts comes down to what you need - I have , in the past, claimed a new suit , shoes, tie etc due to attending an event where these were in my luggage